Arrays and methods for guided cell patterning

ABSTRACT

Guided cell patterning arrays for single cell patterning are disclosed. The arrays include a plurality of cell adhesion sites that are individually isolated on an inert surface. Each cell adhesion site has one or more cell adhesion peptides having affinity to a cell surface receptor. The inert surface is resistant to cell adhesion.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of application Ser. No. 13/846,681, filedMar. 18, 2013, which is a division of application Ser. No. 12/496,730,filed Jul. 2, 2009, which is a continuation of International ApplicationNo. PCT/US2008/050307, filed Jan. 4, 2008, which claims the benefit ofProvisional Application No. 60/883,480, filed Jan. 4, 2007. Eachapplication is incorporated herein by reference in its entirety.

STATEMENT OF GOVERNMENT LICENSE RIGHTS

This invention was made with Government support under Contract No. 5R01GM075095, awarded by the National Institutes of Health and underContract No. EEC9529161, awarded by the National Science Foundation. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

The ability to position and probe a single cell is of great interest infundamental cell biology, cell-based biosensor technologies, medicaldiagnostics, and tissue engineering. Because critical cell-to-celldifferences are lost in average bulk cell measurements, the single cellanalysis with its ability to reveal the response of each individual cellunder stimulation is fundamental to comprehending many biologicalprocesses and mechanisms. Patterning viable single cells on anaddressable array of identical cell hosts, such as an array ofmicroelectrodes with the same physical and chemical properties, wouldaid the statistical analysis of single cell behavior and cell/matrixinteraction. In practical applications, particularly for screening,detection, or sensing systems, microarrays of single cells allow forrapid and inexpensive analysis, require minimal sample volume, andprovide high throughput data acquisition and portability.

Cell patterning, micropatterning of living cells on substrates, hasexperienced a rapid growth in recent years. A number of techniques havebeen developed to produce micro-scale cell patterns. Examples includemicrocontact printing, microfluidic channels, elastomeric stencils, andelastomeric membranes, which involve the delivery of proteins/peptidesto guide cell adhesion or direct deposit of cells on a substrate of asingle material. Cell patterning can also be achieved by tailoringsurfaces to form distinct regions that have adhesive proteins or ligandsto host one or groups of cells with a background inert to proteinabsorption and cell adhesion. Cell patterning can be accomplished viasoft lithography (see, for example, Y. Xia and G. M. Whitesides, Angew.Chem. 110 (5), 568-594, 1998), photochemistry (see, for example, L. M.Tender et al., Langmuir 12:5515-5518, 1996), and photolithographytechniques (see, for example, W. Knoll et al., J. Adv. Biophys.34:231-251, 1997). In these techniques, the patterns are formed eitherby generation of heterogeneous chemistry on a single material or bydeposition of a second material in a certain shape and geometry followedby surface modification to form heterogeneous chemistry.

One of the important applications for the cell patterning is for thedevelopment of cell-based biosensors (CBBs), in which the patternedregions are miniaturized arrays of metal electrodes and the backgroundis an insulate substrate material. Cell-based biosensors are generallyconstructed by interfacing cells to a transducer that converts cellularresponses into signals detectable by electronic or optical devices. CBBsare hybrid systems of biology and device that use cells' abilities todetect, transduce, and amplify very small changes of external stimuli.Cell-based biosensors offer new opportunities for many medicalapplications, including biothreat detection, drug evaluation, pollutantidentification, and cell type determination.

Recent years have witnessed a substantial growth in application ofplanar microelectrode arrays in CBBs because they can be readilyinterfaced with electronic, optical, or chemical detecting means. Majoradvantages of these sensing arrays over conventional biosensors includerapid and inexpensive analyses, smaller sample size requirement, lowsample contamination, high throughput and sensitivity, and portability.Among these sensors, single-cell-based sensors are of particularinterest. With an array of virtually identical single cells as sensingelements integrated with real-time data acquisition technology,single-cell-based sensors can be used to experimentally study cellularpathways without interference from other cells, thereby eliminating theuncertainty incurred by states of neighboring cells. In addition,statistical analysis of cell behavior, a topic extensively pursued incell biology, requires closely identical cell sites, and asingle-cell-based system may ideally serve the purpose.

Despite the encouraging advances made with micropatterning of livingcells on substrates, patterning single cells on a microarray andretaining their viability for a prolonged period of time remain as achallenge. Single cell patterning requires an area for cell adhesion ata size comparable to an individual cell, which is typically 10 to 20 μm,to minimize the probability of a second cell attachment. However,adhesion sites of such small areas tend to suppress cell spread and thusare prone to causing cell death. It was reported that cells could begeometrically switched between growth and apoptosis. Endothelial cellscultured on single islands coated with fibronectin spread and progressedthrough the cell cycle when the island area was larger thanapproximately 40 μm×40 μm, but failed to extend and underwent apoptosiswhen cells were restricted to areas smaller than approximately 20 μm×20μm.

A need exists for devices and methods for patterning single cells thatallow single-cell adhesion while maintaining cellular viability for aprolonged period of time. The present invention seeks to fulfill theseneeds and provides further related advantages.

SUMMARY OF THE INVENTION

The present invention provides guided cell patterning arrays, methodsfor making the arrays, and methods for using the arrays.

In one aspect, the invention provides an array for guided cellpatterning. In one embodiment, the array includes a plurality of celladhesion sites, each site being individually isolated on an inertsurface, wherein each cell adhesion site comprises one or more ligandshaving an affinity to a cell surface receptor; and wherein the inertsurface is resistant to cell adhesion.

In one embodiment, each cell adhesion site further comprises a singlecell immobilized thereto by the interaction of the one or more ligandsand one or more cell surface receptors of the immobilized cell. Inanother embodiment, each cell adhesion site further comprises two ormore cells immobilized thereto by the interaction of the one or moreligands and one or more cell surface receptors of the immobilized cells.

In one embodiment, the inert surface comprises a silicon surface havingpolyalkylene oxide moieties covalently attached thereto. In anotherembodiment, the inert surface comprises an oxidized silicon surfacehaving polyalkylene oxide moieties covalently attached thereto.

In another aspect of the invention, methods for making an array of celladhesion sites is provided. In one embodiment, the method includes:

-   -   (a) providing a metal-patterned silicon substrate having an        array of metal surfaces disposed on a silicon surface;    -   (b) forming a self-assembly monolayer on each metal surface to        provide an array of monolayers disposed on the silicon surface;    -   (c) passivating the silicon surface by covalently coupling        polyalkylene oxide moieties to the silicon surface to provide a        surface resistant to cell adhesion isolating each self-assembly        monolayer of the monolayer array; and    -   (d) attaching a plurality of ligands to each self-assembly        monolayer to provide an array of cell adhesion sites.

In one embodiment, the method further includes immobilizing a singlecell at each cell adhesion site through the interaction of the ligandsand one or more cell surface receptors of the cell. In anotherembodiment, the method further includes immobilizing two or more cellsat each cell adhesion site through the interaction of the ligands andone or more cell surface receptors of the cells.

In another aspect, the invention provides methods for analyzing aplurality of single cells immobilized in an array. In one embodiment,the method includes:

-   -   (a) subjecting one or more cells individually immobilized in an        array to a stimulus to provide an array comprising individually        treated cells, the array comprising a plurality of cell adhesion        sites, each site isolated on an inert surface, wherein each cell        adhesion site comprises a single cell immobilized thereto by the        interaction of one or more ligands attached to the site and one        or more cell surface receptors of the immobilized cell, and        wherein the inert surface is resistant to cell adhesion; and    -   (b) individually addressing one or more of the treated cells to        measure the effect of the stimulus on the treated cells.

In one embodiment, individually addressing one or more of the treatedcells comprises individually addressing the treated cells optically. Inanother embodiment, individually addressing one or more of the treatedcells comprises individually addressing the treated cells electrically.

In one embodiment, the stimulus is a therapeutic drug compounds. Inanother embodiment, the stimulus is a toxin.

In a further aspect, the invention provides an array for guided cellpatterning having a passivated silicon oxide surface. In one embodiment,the array includes a plurality of individually immobilized cellsisolated on an inert surface resistant to cell adhesion, wherein theinert surface comprises a silicon oxide surface having polyalkyleneoxide moieties covalently coupled thereto, and wherein the silicon oxidesurface comprises from about 40% to 65% by weight Si, from about 5% toabout 20% by weight SiO_(x<2), and from about 20% to about 40% by weightSiO₂.

In one embodiment, each cell is immobilized through the interaction ofone or more ligands and one or more cell surface receptors of theimmobilized cell.

In one embodiment, the silicon oxide surface comprises from about 50% to60% by weight Si, from about 10% to about 15% by weight SiO_(x<2), andfrom about 25% to about 35% by weight SiO₂. In another embodiment, thesilicon oxide surface comprises about 58% by weight Si, about 12% byweight SiO_(x<2), and about 30% by weight SiO₂.

In another aspect of the invention, methods for making an array forguided cell patterning having a passivated silicon oxide surface isprovided. In one embodiment, the method includes:

-   -   (a) providing a metal-patterned silicon substrate having an        array of metal surfaces disposed on a silicon surface;    -   (b) exposing the substrate to an oxide etch to remove native        oxide from the silicon oxide surface to provide a native oxide        depleted silicon surface;    -   (c) oxidizing the native oxide depleted silicon surface with an        oxidizing agent to provide a silicon oxide surface; and    -   (d) passivating the silicon oxide surface by covalently coupling        polyalkylene oxide moieties to the silicon oxide surface to        provide a surface resistant to cell adhesion isolating each        metal surface of the metal surface array.

In one embodiment, the method further includes forming a self-assemblymonolayer on each metal surface to provide an array of monolayersisolated on the silicon oxide surface. In another embodiment, the methodfurther includes attaching a plurality of ligands to each self-assemblymonolayer to provide an array of cell adhesion sites.

In one embodiment, the further includes immobilizing a single cell ateach cell adhesion site through the interaction of the ligands and oneor more cell surface receptors of the cell. In another embodiment, themethod further includes immobilizing two or more cells at each celladhesion site through the interaction of the ligands and one or morecell surface receptors of the cells.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings.

FIG. 1A is a schematic illustration of a representative cell adhesionarray of the invention; FIG. 1B is a schematic illustration of arepresentative cell adhesion array of the invention showing arraycomponents; and FIG. 1C is a schematic illustration of the array of FIG.1B showing adhered cells.

FIGS. 2A-2E compare the high-resolution Si_(2p) spectra of siliconsubstrates coated with native oxide, dry oxide, and wet oxide; and FIGS.2F-2H compare the high-resolution C_(1s) spectra of PEG on siliconsubstrates coated with native oxide, dry oxide, and wet oxide.

FIGS. 3A-3F are the fluorescent images of the fibronectin-Cy3 conjugateadsorbed on surfaces patterned with gold electrodes. FIGS. 3A and 3Bshow the fluorescent images of the fibronectin-Cy3 conjugate adsorbed onunmodified and PEG-modified surfaces with native oxide on a siliconbackground, respectively; FIGS. 3C and 3D show the fluorescent image ofthe fibronectin-Cy3 conjugate adsorbed on unmodified and PEG-modifiedsurfaces with wet oxide on a silicon background, respectively; and FIGS.3E and 3F show the fluorescent image of the fibronectin-Cy3 conjugateadsorbed on unmodified and PEG-modified surfaces with dry oxide on asilicon background, respectively. Scale bar is 60 μm.

FIGS. 4A-4I are the DIC reflectance microscopic images of macrophagecells cultured on gold-patterned silicon surfaces up to 10 days. FIGS.4A, 4B, and 4C are images of macrophage cells cultured on gold-patternedsilicon surfaces with native oxide on silicon background for 3, 7, and10 days, respectively; FIGS. 4D, 4E, and 4F are images of macrophagecells cultured on gold-patterned silicon surfaces with wet oxide siliconbackground for 3, 7, and 10 days, respectively; and FIGS. 4G, 4F, and 4Iare images of macrophage cells cultured on gold-patterned siliconsurfaces with dry oxide on silicon background for 3, 7, and 10 days,respectively. Scale bars are 60 μm in all low-magnification images and20 μm in all high-magnification images (insets in all images).

FIGS. 5A, 5B, and 5C are schematic representations of gold squarescoated with proteins or peptides. FIG. 5A shows gold squares coated withfibronectin with multiple types of cell adhesion sequences; FIG. 5Bshows gold squares coated with physically adsorbed KREDVY and REDVY; andFIG. 5C shows gold squares coated with covalently coupled KREDVY withREDVY.

FIG. 6 compares the grazing angle FTIR absorption spectra of surfaceswith the coating: (a) NHS ester terminated self-assembly monolayer; (b)physically adsorbed REDVY on the self-assembly monolayer; (c) covalentlycoupled KREDVY on self-assembly monolayer; (d) covalently coupledfibronectin on the self-assembly monolayer.

FIGS. 7A, 7B, and 7C show the optical micrographs of HUVE cellspatterned on gold electrodes of silicon oxide substrates with goldelectrodes coated with fibronectin, physically adsorbed REDVY, andcovalently coupled KREDVY, respectively. The insets show a magnifiedcell image for each case to reveal the cell morphology.

FIGS. 8A-8D are the fluorescent confocal microscopic images of HUVEcells adhered on gold patterns coated with fibronectin (left), REDVYpeptide (middle), and KREDVY peptide (right). FIG. 8A shows thetrichromatic fluorescence images of the cells stained with DAPI,immunostain (monoclonal anti-vinculin-FITC conjugate), and ALEXA FLUOR594 phalloidin dye for nuclei, F-actin, and vinculin, respectively. FIG.8B shows the images of the channel for nuclei; FIG. 8C shows the imagesof the channel for vinculin; and FIG. 7D shows the images of the channelfor actin.

FIGS. 9A-9D are the fluorescence images of HUVE cells cultured ongold-patterned silicon oxide substrates. FIG. 9A is the image of HUVEcells cultured on gold-patterned silicon oxide substrates with goldsquares coated with fibronectin at 10× objectives; FIG. 9B is the imageof HUVE cells cultured on gold-patterned silicon oxide substrates withgold squares coated with physically adsorbed REDVY at 10× objectives;and FIGS. 9C and 9D are the images of HUVE cells cultured ongold-patterned silicon oxide substrates with gold squares coated withcovalently coupled KREDVY at 10× objective and 5× objective,respectively. FIG. 9C was captured from FIG. 9D in the area surroundedby the white rectangle. Scale bars are 60 μm in all images.

FIGS. 10A-10F are the fluorescence images of HUVE cells on the goldpatterns immobilized with proteins or peptides after 7 days of celladhesion. FIGS. 10A and 10D are images of cells on the gold patternsimmobilized with fibronectin; FIGS. 10B and 10E are images of cells onthe gold patterns immobilized with REDVY peptide; and FIGS. 10C and 10Fare images of cells on the gold patterns immobilized with KREDVYpeptide. FIGS. 10A, 10B, and 10C show the apoptotic cells in fluorescegreen; and FIGS. 10D, 10E, and 10F show necrotic cells in fluoresce red.Live cells show little or no fluorescence. Images were taken fromtriplicate substrates for each type of surfaces.

FIGS. 11A-11D are optical DIC images of macrophage cells cultured onfibronectin-coated electrodes after culture and exposed to LPS for 21hours. FIG. 11A shows the control cells with no LPS treatment; FIG. 11Bshows the cells treated with LPS at concentration of 0.1 μg/mL; FIG. 11Cshows the cells treated with LPS at concentration of 1.0 μg/mL; and FIG.11D shows the cells treated with LPS at concentration of 10 μg/mL.

FIGS. 12A, 12C, and 12E are optical DIC images of 100 μm² electrodeshosting a single macrophage cell, double cells, and triple cells,respectively, after treatment with 1 μg/mL LPS for 21 hours; and FIGS.12B, 12D, and 12F are real-time synchrotron IR spectra of a single cell,double cells, and triple cells, respectively, before and after treatmentof LPS.

FIG. 13A shows the real-time synchrotron FTIR spectra taken from singlemacrophage cell patterned on gold electrode with an area of 100 μm²; andFIGS. 13B-13E are optical DIC images of macrophage cells. Specifically,FIG. 13B shows the image of cells not treated with LPS; FIGS. 13C-13Eshow the images of cells treated with LPS at concentrations of 0.1μg/mL, 1.0 μg/mL, and 10 μg/mL for 21 hours, respectively.

FIGS. 14A and 14B are optical DIC images of a single macrophage cellpatterned on a gold electrode with a surface area of 100 μm², culturedwith 10 μg/mL LPS for 21 hours, and stained. FIG. 14A are cell imagesstained with Annexin V for 15 min; and FIG. 14B are cell images stainedwith propidium iodide for 15 min.

FIG. 15A shows the real-time synchrotron IR spectrum of a single cellresponse to LPS (1.0 μg/mL) over time; FIGS. 15B, 15C, and 15D areoptical DIC images of a single macrophage cell with no LPS treatment,with LPS (1.0 μg/mL) treatment for 3.5 hours, and with LPS (1.0 μg/mL)treatment for 21 hours, respectively.

FIGS. 16A-16D compares FTIR spectra of single macrophage cells onelectrode of three different sizes: 25 μm², 100 μm², and 400 μm². FIGS.16A and 16B show FTIR spectra acquired by synchrotron FTIR at the wavenumber of 3200-3600 cm⁻¹ and 1200-1800 cm⁻¹, respectively; and FIGS. 16Cand 16D show FTIR spectra acquired by conventional FTIR with aperturesize as 90 μm×90 μm at the wave number of 2600-4000 cm⁻¹ and 1200-1800cm⁻¹, respectively.

FIG. 17 shows an array of single DAOY cells patterned on 20 μm goldsquares covalently bound with Lys-Arg-Gly-Asp (KRGD) peptide.

FIG. 18 shows an array of single 9 L (rat glioma) cells patterned on 20μm gold squares covalently bound with chlorotoxin.

FIG. 19 shows an array of single 6 L (glioma) cells patterned on 20 μmgold squares covalently bound with Lys-Arg-Gly-Asp (KRGD) peptide.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides guided cell patterning arrays, methodsfor making the arrays, and methods for using the arrays. The arrays areuseful for single cell patterning and provide improved patterningprecision, selectivity, stability, and reproducibility.

In one aspect, the present invention provides cell-patterning arrays forguided cell patterning. In one embodiment, the array has a plurality ofcell adhesion sites, each site being individually isolated on an inertsurface that is resistant to cell adhesion. In the array, each celladhesion site includes one or more ligands that have an affinity to acell surface receptor on a cell. Through the use of the ligandsimmobilized at each site, the arrays of the invention can be used toselectively immobilize desired cells (i.e., ligands can be selected forthe site depending on their affinity to cell surface receptors on thecells of interest to be immobilized). The array's inert surfaceresistant to cell adhesion provides for precise cell immobilization(i.e., cell immobilization only at the adhesion sites).

As used herein, the term “cell adhesion sites” refers to sites on thearray that are capable of cell attachment and supporting cell growth. Asused herein, the term “inert surface” refers to a surface that iscapable of resisting cell adhesion (e.g., by blocking or reducingnonspecific protein interaction). Each individual cell adhesion site isisolated by a portion of inert surface thereby defining an array of celladhesion sites.

The array's inert surface is a silicon surface that has been passivated(i.e., rendered resistant to cell adhesion) by the attachment (e.g.,covalent coupling) of polyalkylene oxide moieties. The polyalkyleneoxide moieties may form a self-assembly monolayer on the siliconsurface.

In one embodiment, the inert surface is prepared from silicon substratehaving a silicon oxide surface. As used herein, “silicon oxide surface”refers to an oxidized surface of a native oxide depleted siliconsurface. The silicon oxide surface is prepared by removing native oxidefrom a silicon substrate surface to provide a depleted native oxidesurface followed by oxidizing the depleted native oxide surface. In oneembodiment, the silicon oxide surface includes from about 40% to 65% byweight Si, from about 5% to about 20% by weight SiO_(x<2), and fromabout 20% to about 40% by weight SiO₂. In another embodiment, thesilicon oxide surface includes from about 50% to 60% by weight Si, fromabout 10% to about 15% by weight SiO_(x<2), and from about 25% to about35% by weight SiO₂. In a further embodiment, the silicon oxide surfaceincludes from about 58% by weight Si, about 12% by weight SiO_(x<2), andabout 30% by weight SiO₂. As noted above, the Si/SiO_(x<2)/SiO₂composition of the silicon oxide surface can be achieved by removing thenative oxide from a silicon surface to provide a native oxide depletedsilicon surface followed by oxidizing the native oxide depleted siliconsurface with dry oxygen flow at about 300° C. to about 500° C. for fromabout 5 to about 24 hours. The preparation of a silicon oxide surfaceand its conversion to an inert surface of the array of the invention isdescribed below.

The arrays of the invention are used to precisely immobilize selectcells. Each cell adhesion site can immobilize one or more cells theretoby the interaction of the one or more ligands and one or more cellsurface receptors of the immobilized cells. In one embodiment, each celladhesion site includes a single cell. In this embodiment, the array isan array of single immobilized cells. Alternatively, each cell adhesionsite can include two or more immobilized cells.

Any cell that is viable under cultured conditions can be used in thepresent invention. Representative cells that can be immobilized to thecell adhesion sites of the arrays of the invention described belowinclude human umbilical cord vein endothelial (HUVE) cells, DAOY cells,glioma cells, and macrophages. Other exemplary cells advantageouslyimmobilized by the arrays and methods of the invention include stemcells, bone cells, muscle cells, and nerve cells, among others. It willbe appreciated that the type and nature of the cell immobilized by thearrays and methods of the invention are not limited to those describedherein. Any cell having a cell surface receptor that can be immobilizedthrough the interaction of the ligands attached to the cell adhesionsite can be advantageously immobilized by the methods described hereinto provide the cell-patterned arrays of the invention.

The cell adhesion sites include one or more ligands attached at the siteand that act to selectively immobilize cells. The ligand useful for usein a particular array will be selected depending on the cell to beimmobilized. Each cell adhesion site presents one or more ligands forimmobilizing cells. The ligands are attached to a self-assembledmonolayer (e.g., polyalkylene), which is attached to a metal surface onthe silicon substrate making up the array platform. The ligands can beadsorbed or covalently coupled to the monolayer.

As used herein, the term “ligand” refers to a substance that binds in ahighly specific manner to its cell surface receptor. The term “cellsurface receptor” refers to a protein on the cell membrane that binds tothe ligand.

In one embodiment, the ligand is a cell adhesion peptide. Representativecell adhesion peptides that can be incorporated into the cell adhesionsites of the arrays of the invention described below includeLys-Arg-Glu-Asp-Val-Tyr (SEQ ID NO:1) (KREDVY) (ligand for humanumbilical cord vein endothelial (HUVE) cells), Lys-Arg-Gly-Asp (SEQ IDNO: 2) (KRGD) (ligand for DAOY cells), and Arg-Gly-Asp (RGD) (ligand forcartilage, muscle, and human mesenchymal stem cells). In otherembodiments, the cell adhesion peptide is a fragment of fibronectin or afragment of other cell adhesion proteins.

In one embodiment, the ligand is a peptide. Representative peptidesuseful as ligands in the invention include chlorotoxin, a 36 residuepeptide with high affinity to cells expressing MMP-2 receptors (ligandfor rat glioma cells), and nerve growth factor (NGF) (ligand for PC 12neuronal cells).

In another embodiment, the ligand is a cell adhesion protein.Fibronectin, a cell adhesion protein, is not a ligand for arrays andmethods of the invention that do not include a silicon oxide surface asdefined herein. Fibronectin is a ligand for arrays and methods of theinvention that do include a silicon oxide surface as defined herein.

It will be appreciated that the type and nature of the ligands useful inthe arrays and methods of the invention are not limited to thosedescribed herein.

A representative array of the invention uses the cell adhesion peptideLys-Arg-Glu-Asp-Val-Tyr (KREDVY) as the ligand to immobilize humanumbilical cord vein endothelial (HUVE) cells. FIG. 7C shows an array ofHUVE cells patterned on gold squares covalently bonded withLys-Arg-Glu-Asp-Val-Tyr peptide. In another embodiment, the celladhesion peptide Arg-Glu-Asp-Val-Tyr (SEQ ID NO: 3) (REDVY) is theligand for immobilizing human umbilical cord vein endothelial cells.FIG. 7B shows an array of HUVE cells patterned on gold squaresphysically adsorbed with Arg-Glu-Asp-Val-Tyr peptide. In anotherembodiment, the cell adhesion peptide Lys-Arg-Gly-Asp (KRGD) is theligand for immobilizing DAOY cells. FIG. 17 shows an array of singleDAOY cells patterned on 20 μm gold squares and having ligandLys-Arg-Gly-Asp covalently coupled at the adhesion site. In anotherembodiment, chlorotoxin is the ligand for immobilizing 9 L (rat glioma)cells. FIG. 18 shows an array of single 9 L (rat glioma) cells patternedon 20 μm gold squares and having ligand chlorotoxin covalently coupledat the adhesion site. In another embodiment, Lys-Arg-Gly-Asp is theligand for immobilizing 6 L (glioma) cells. FIG. 19 shows an array ofsingle 6 L (glioma) cells patterned on 20 μm gold squares and havingLys-Arg-Gly-Asp covalently coupled at the adhesion site. Referring tothese figures, the cells are spread over the cell adhesion sites and theinert surface (polyethylene oxide-modified surface) shows no celladherence.

A representative array of the invention is illustrated schematically inFIG. 1A. Referring to FIG. 1A, array 100 includes silicon substrate 2having inert surface 4 individually isolating a plurality of celladhesion sites 6. The components of a representative array areillustrated schematically in FIG. 1B. Referring to FIG. 1B, celladhesion sites 6 terminate with ligands 14 attached to linkers 12 thatmake up self-assembly monolayers at each site. Linkers 12 are coupled togold surfaces 10. Titanium surfaces 8 are intermediate gold surfaces 10and substrate 2. Cell adhesion sites 6 are individually isolated (i.e.,isolated from each other) by inert surface 4 having polyalkylene oxidemoieties 18 coupled to substrate 2. FIG. 1C illustrates a representativesingle cell patterned array of the invention. Single cells 16 are shownimmobilized to sites 6 through ligands 14.

In another aspect, the present invention provides methods for making acell-patterning array. In one embodiment, the method includes:

-   -   (a) providing a metal-patterned silicon substrate having an        array of metal surfaces disposed on a silicon surface;    -   (b) forming a self-assembly monolayer on each metal surface to        provide an array of monolayers disposed on the silicon surface;    -   (c) passivating the silicon surface by covalently coupling        polyalkylene oxide moieties to the silicon surface to provide a        surface resistant to cell adhesion isolating each self-assembly        monolayer of the monolayer array; and    -   (d) attaching a plurality of ligands to each self-assembly        monolayer to provide an array of cell adhesion sites.

Each cell adhesion site can include one or more cells immobilizedthereto by the interaction of the one or more ligands and one or morecell surface receptors of the immobilized cells. In one embodiment, eachcell adhesion site includes a single cell. In this embodiment, the arrayis an array of single immobilized cells. Alternatively, each celladhesion site can include two or more immobilized cells.

The silicon surface can be passivated by covalently couplingpolyalkylene oxide moieties to the silicon surface. As used herein, theterm “passivating” refers to rendering a surface resistant tononspecific protein adsorption and cell adhesion.

Silanated polyethylene oxide can be used to react with the siliconsurface attaching the polyethylene oxide moiety to the silicon surface.In one embodiment, the polyalkylene oxide moieties can be coupled to thesurface by exposing the silicon surface to a reactive silaneterminated-polyalkylene oxide. Polyethylene oxide moieties can becovalently attached to the silicon surface forming a self-assembly layer(SAM). The choice of catalyst, solvent type, polyethylene oxidechain-length, polyethylene oxide concentration, humidity, temperature,and reaction time can be used to optimize the conditions for forming auniform monolayer of polyethylene oxide polymers on the oxide surface.In one embodiment, a low molecular weight silanated polyethylene oxidepolymer, methoxy-PEG silane (M-PEG-silane, molecular weight=460-590Dalton, 2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane), was reactedwith the silicon surface.

The metal-patterned silicon substrate can comprise a p-type siliconsubstrate with (100) orientation having an array of metal squarespatterned thereon. The metal square can be made from any suitable metal,such as gold, platinum, or silver. In one embodiment, the metal squareis a gold square. A titanium square can be deposited on the substrateprior to depositing the gold square. The thickness of the gold squarecan be from about 50 nm to about 500 nm. The thickness of the titaniumlayer can be from about 2 nm to about 50 nm.

The metal square can be coated with a self-assembly monolayer. In oneembodiment, the method further includes forming a self-assemblymonolayer on each metal surface by reacting the metal surfaces with athiol-terminated alkanoic acid to provide a carboxylic acid-terminatedmonolayer. In one embodiment, the thiol-terminated alkanoic acid is aC3-C25 alkanoic acid. In one embodiment, the thiol-terminated alkanoicacid is 3-mercaptopropionic acid. In another embodiment, thethiol-terminated alkanoic acid is 11-mercaptoundecanoic acid.

Ligands (e.g., cell adhesion peptides) are attached to monolayer toprovide cell adhesion sites. The ligands can be covalently coupled tothe self-assembly monolayer through amide bonds formed by the reactionbetween the terminal carboxylic acid groups on the self-assemblymonolayer and the amino groups of the ligands (e.g., the lysine aminofor a cell adhesion peptide). Alternatively, the ligands may bephysically adsorbed to the self-assembly monolayer.

The preparation of a representative cell-patterning array of humanumbilical cord vein endothelial (HUVE) cells is described in Example 1.Endothelial cells play an important role in angiogenesis and tissuerepair, and have a broad range of application in detection of bacteria,virus, and toxins. Endothelial cells serve as major barriers separatingthe blood from tissue compartments whose interaction with bacteriadefines the course of invasive infections and inflammatory responses.

A patterned gold/silicon oxide-based cell-patterning platform wasprepared according to the method of the present invention. Siliconsubstrates were cleaned with piranha at 120° C. for 10 minutes, dippedin HF and rinsed with DI water thoroughly. A layer of positivephotoresist was then coated on the surface and an array of gold squarewas patterned on the silicon oxide substrate by conventionalmicrofabrication. Specifically, a layer of titanium (Ti) was depositedonto photoresist-developed substrates. A gold film was subsequentlydeposited onto the titanium. The photoresist was dissolved and theremaining metal film was lifted off. After lift-off, the surfaces wereexposed to buffered oxide etch and rinsed with water to remove thenative oxide on silicon regions followed by oxidation with dry air toprovide dry oxide surfaces.

The gold squares were chemically modified by reacting COOH-terminatedalkyl thiols with the gold layer to afford a surface coated with aCOOH-terminated self-assembly monolayer (SAM). The dry oxide surface waspassivated with a PEG coating to afford the inert region, which isresistant to nonspecific protein adsorption and cell adhesion. Theplatforms were then exposed to (a) fibronectin, (b) Arg-Glu-Asp-Val-Tyr(REDVY), or (c) Lys-Arg-Glu-Asp-Val-Tyr (KREDVY), which are used tomediate single cell adhesion and maintain cellular viability. Threeplatforms with cell adhesion regions containing covalently boundfibronectin and KREDVY and physically adsorbed REDVY were obtained. Theplatform having fibronectin as an adhesion protein and the platform withphysically bound REDVY are used as control platforms.

The cell adhesion sites, gold squares coated with fibronectin, REDVY, orKREDVY peptide, are schematically illustrated in FIGS. 5A, 5B, and 5C,respectively. Fibronectin and KREDVY with lysine residues (K) containingε-amino groups were covalently bound to the COOH-terminated SAM on thegold squares. This bonding was formed by activation of the terminalcarboxylate group with an N-hydroxysuccinimide (NHS) ester intermediatefollowed by the displacement of the NHS group by the lysine residue ofproteins or peptides. Lacking a reactive lysine residue, REDVY wasphysically adsorbed onto the gold square. These processes produced threepatterned surfaces of different surface chemistries and cell-bindingnature, each with an array of cell-adhesive regions (gold squares) on anon-adhesive background (inert regions, passivated silicon oxide) asshown in FIGS. 5A, 5B, and 5C.

The surface chemistries and binding properties of three platforms werecharacterized by reflectance FTIR spectroscopy. FIG. 6 shows the IRspectra acquired from the surfaces coated with (a) SAM-NHS, (b)SAM-NHS-REDVY, (c) SAM-NHS-KREDVY, and (d) SAM-NHS-fibronectin. Spectrum(a) has characteristic bands at 1078, 1222, 1370, and 1741 cm⁻¹,obtained from the surface modified with SAM-NHS, a monolayer on whichthe protein or peptides were further immobilized. The high intensityband at 1741 cm⁻¹ is attributed to asymmetric stretch of NHS carbonyls,and indicates successful covalent binding of NHS with the underlyingalkyl thiol SAM. The amide I and amide II peaks in spectrum (b) indicatethe presence of associated peptide bonds on the surface. Physicaladsorption of REDVY on the surface is confirmed by the presence ofunreacted NHS groups with spectrum bands corresponding to vas (CNC) andv (NCO) of NHS at 1222 cm⁻¹ and 1078 cm⁻¹ that would be otherwise absentdue to the chemical bonding. Successful covalent binding between thelysine residue of KREDVY and NHS monolayer is characterized by thepresence of the amide I and amide II bands at 1653 and 1543 cm⁻¹(spectrum (c)), the significant reduction of the intensity of carbonylpeak at 1741 cm⁻¹ as well as the absence of bands at 1078, 1222, and1370 cm⁻¹ that correspond to NCO stretch, asymmetric CNC stretch, andsymmetric CNC stretch of the NHS, respectively. However, the continuedpresence of a small peak at 1741 cm⁻¹ suggests the incomplete conversionof SAM carboxylic groups to amides. Spectrum (d), acquired from thefibronectin-modified surface, confirms the covalent interactions offibronectin with NHS-SAM. A full conversion of carboxylic groups toamides is characterized by the complete absence of peaks at 1741 cm⁻¹and 1222 cm⁻¹ and the presence of strong amide I and amide II peaks. Thehigh intensities of amide I and amide II bands indicate a high densityof peptides on the surface, presumably due to the large amount ofpeptides in fibronectin.

To create microarrays of cell patterns, HUVE cells were cultured oncell-patterning platforms with fibronectin or peptide in standardculture media for 18 hours. After cell culture, the cells were fixed andexamined using differential interference contrast (DIC) reflectanceoptical microscopy. Both REDVY and KREDVY contain tetrapeptide REDVsequence specific to α4β1 receptors of HUVE cells. Fibronectin has atleast two types of cell binding sequences for HUVE: the Arg-Gly-Asp(RGD) that would bind to the α5β1 and αvβ3 integrin receptors, and thetetrapeptide REDV that would bind to α4β1 receptors. When immobilized onthe COOH-terminated gold surfaces, these cell adhesion protein andpeptides would exhibit different binding domain configurations as shownconceptually for a gold-based cell adhesive region in FIGS. 5A-5C.Because fibronectins are long-chain molecules, they would bind to thesurface with random orientation of RGD and REDV domains as shown in FIG.5A, whereas short-chain REDVY and KREDVY peptides would exhibit muchmore ordered distribution and/or orientation as shown in FIGS. 5B and5C. Additionally, the KREDVY peptide would exhibit a more uniformmolecular orientation as a result of its covalent bonding with theunderlying SAM layer and thus provide more uniform binding domains.FIGS. 7A, 7B and 7C show the optical images of HUVE cells adhered on thegold-based cell adhesive regions with fibronectin, REDVY, and KREDVY,respectively. The cells were seen to adhere mainly on the sites ofgold-based cell adhesive regions, indicating a high degree of cellselectivity for all three platforms. The cell adhesive regions withbound fibronectin exhibited a pattern of multiple-cell binding on goldsquares and slight nonspecific cell adhesion onto the siliconoxide-based inert regions around the cell adhesive regions. This is notsurprising in light of the multiple types of cell binding sequences andrandomly oriented binding domains of long-chain fibronectin protein.Cell adhesion, the morphology of adhered cells, and the extent of cellspreading are dictated by the availability, conformation, anddistribution of cell binding domains. When a cell approaches agold-based cell adhesive region covered with fibronectin proteinmolecules, it is confronted with binding domains of different types andorientations. It is conceivable that only a portion of the surface-boundprotein molecules and a portion of peptide sequences in those moleculesare involved in the cell binding process, presumably those cell bindingsequences in the molecules that are oriented roughly at the samedirection. Thus, the “effective” binding domains on a gold-based celladhesive region for the adhesion of this particular cell can be only asmall fraction of those physically presented on the gold square. A largenumber of cell binding sequences in fibronectin does not necessarilyresult in a large number of effective binding domains, due to themolecules bound in a random orientation to the surface. The spreading ofthe adhered cell over the gold-based cell adhesive region can also behindered by the limited number of effective binding domains. Instead,the probability of binding a second cell to the same cell adhesiveregion is increased due to availability of free binding domains ofdifferent types and orientations.

Cells attached on the platform with physically adsorbed REDVY peptideexhibited a pattern with most of gold-based cell adhesive regionshosting one cell (FIG. 7B), but with apparently low cell coverage (thenumber of cell adhesive regions occupied by cells versus the totalnumber of cell adhesive regions). In addition, the cells barely spreadacross the gold squares. Two possibilities may account for this low cellcoverage, both because of the weak physical binding between the REDVYpeptide molecules and underlying SAM-NHS layer: (1) the detachment ofREDVY peptide from the gold surface, resulting in an insufficient numberof REDVY peptides available on the gold square-based cell adhesiveregion to bind a cell; (2) the detachment of the cell-REDVY conjugatesfrom the gold square-based cell adhesive region after the cell bound tothe REDVY peptides. Low cell spreading on the REDVY-modified platformcan also be attributed to the physical binding of REDVY molecules. Cellspreading or migration requires the dynamic formation and dispersal ofcell contacts with the extracellular matrix. For receptor-mediated celladhesion, cell spreading on a surface is driven by the traction forceset by adhesive molecules peripheral to the initial focal contactsbetween the cell and surface. As cell spreading proceeds, whichcontinually stretches the cell and increases force, greater force isrequired to cause increased cell spreading. A straightforwardexplanation is that the physically-adsorbed REDVY molecules are unlikelyto sustain such traction; instead, they are prone to be detached fromthe surface by the contractile force of the cell, resulting in thedetachment of the REDVY molecules from the surface. Although thephysically adsorbed REDVY peptides do not form robust binding with theunderlying SAM, they do provide more effective and uniformly distributedbinding sites than fibronectin due to their small molecular size. Thus,the majority of REDVY molecules on the gold square-based cell adhesiveregion would participate in binding of the cell if they are not detachedfrom the surface. Thus, after a cell has been bound, there would be toofew binding domains left for binding of a second cell.

On the platform with chemically bound short KREDVY peptide, highlyspecific single-cell adhesion and higher cell coverage were observed(FIG. 7C). As shown in FIG. 7C, this platform has the most uniform androbust cell binding sites as a result of the covalent binding of KREDVYmolecules on the platform. In addition to having all the advantageousproperties of the REDVY-modified platform for cell binding, thecovalently bound KREDVY is less susceptible to detachment. The fullyspread cell morphology suggests that most of the available bindingdomains on the gold square-based cell adhesive region have participatedin the cell binding, and that the covalently bound KREDVY peptideprovides sufficient traction force for cell spreading. Thus, once a cellis bound to the gold square-based cell adhesive region, there wouldremain insufficient free space or binding domains for adhesion of asecond cell. Small peptides have additional advantages over proteins inthat they are less susceptible to cellular proteolysis and thermaldegradation, and thus most of their active domains are available forcell adhesion.

The cell adhesion models shown in FIGS. 5A-5C were further validated bycell stain assays. Cells adhered on the substrates were fixed,permeabilized, and stained with DAPI (blue), immunostain containing antivinculin-FITC (green), ALEXA FLUOR 594 phalloidin dye (red) to revealthe distributions of nuclei, cell-surface focal contacts, and cellularactin filaments, respectively. FIGS. 8A-D show the fluorescence imagesof cells adhered on the gold squares coated with fibronectin (left),REDVY (middle), and KREDVY (right). FIG. 8A is the overlay of images ofFIG. 8B (cell nuclei), FIG. 8C (focal contacts), and FIG. 8D (actinfilaments). Cells on the three surfaces generated differentmorphological, cytoskeletal, and adhesion signals. The nucleus images inFIG. 8B show that two cells were attached on the gold square-based celladhesion regions with chemically bound fibronectin (left) and singlecells on the gold square-based cell adhesion regions with physicallyadsorbed REDVY (middle) or chemically bound KREDVY (right). FIG. 8Creveals that two cells on the fibronectin modified platform (left)formed fewer focal contacts (per cell) with the substrate than did cellson the KREDVY modified platforms (right), even though many cell bindingsequences (hence cell adhesion domains) are available in fibronectinmolecules. This supports the hypothesis noted above that only a portionof the available binding sequences in fibronectin participated inbinding of a specific cell. The availability of cell binding domains,plus partial coverage of the gold square-based cell adhesion regions bya first-arrival cell, allows for the adherence of a second cell via adifferent type and/or orientation of binding domains. Spreading of thecells on the fibronectin-modified surface, as shown by the actinfilaments (FIG. 8D, left), extended beyond the gold square boundary.

The cell on the REDVY-modified surface has fewer focal contacts with thesubstrate than the cell on the surface with KREDVY, because thephysically adsorbed peptide is prone to detachment from the surface. Thecell is not fully spread to cover the entire gold square-based celladhesion region for the same reason as identified by its actin filamentimage shown in FIG. 8D, middle. The cell on the KREDVY modified surfaceformed the densest and most uniform focal contacts with the substrate,and the short peptide molecules confined the cell spreading to the celladhesion region, as shown in the cell filament image FIG. 8D, right,leaving no additional cell binding domains or space for adhesion of asecond cell. The dense KREDVY peptide molecules also facilitatepreferential attachment of actin filaments terminated at the edges ofcell adhesion region and vinculin proteins concentrated on the domainsaround cell nucleus and the edges of the cell adhesion region, resultingin a fully spread cellular morphology.

Although covalently bound KREDVY peptides considerably increased theprobability of single cell adhesion as compared to protein-mediated celladhesion, multiple cells were still seen to adhere to individual goldsquare-based cell adhesion regions on a small number of them. Thus,quantification of single cell coverage is necessary for understandingthe capability of this peptide-mediated cell adhesion process. HUVEcells were cultured on fibronectin and peptide modified platforms instandard culture media for 18 hours and stained with DAPI for cellnuclei to emit blue fluorescence. Fluorescence microscopy was used toidentify single or multiple cells on gold square-based cell adhesionregions. Cell coverage and single cell population for each type ofsurface were quantified from 378 gold square-based cell adhesion regions(3 samples×2 areas of interest with 63 cell adhesion regions per area).Cell coverage was calculated from the ratio of gold square-based celladhesion regions covered with cells to the total number of the celladhesion regions in the area of interest, and single cell ratio wasobtained by dividing the number of gold square-based cell adhesionregions covered with single cells to the total number of cell adhesionregions covered with cells (single or multiple). Exemplary images ofadhered cells on three model platforms are shown in FIGS. 9A-D, andquantification results are shown in Table 1. As expected, thefibronectin and KREDVY modified surfaces have higher cell coverage thanthe REDVY modified surface due to larger number binding domainsavailable on the former two surfaces than on the latter. Both peptidemodified surfaces have much higher single cell ratios than thefibronectin modified surface. Clearly, the KREDVY modified surface isthe best candidate for single cell patterning in light of both cellcoverage and single cell ratio. As a side note, for electricalrecording, it is easy to distinguish between single and multiple cells,allowing either type of site to be observed selectively.

TABLE 1 Quantification of HUVEs on gold-patterned silicon substratesmodified with fibronectin, REDVY, and KREDVY. % Ratio of single-cellSurface Coating % Cell Coverage sites/total cell sites Fibronectin 83.47± 3.20 27.41 ± 3.64 REDVY 56.28 ± 3.03 62.25 ± 3.23 KREDVY 78.40 ± 3.9672.17 ± 2.49

Retaining cell viability after cell adhesion is essential for cellbiology studies and biomedical applications such as cell-based sensorsand drug screening microarrays. The viability of cells patterned on thethree model platforms were studied with an apoptosis assay noted inEXAMPLE section. FIGS. 10A-F show fluorescence images of HUVE cells oncell patterning platforms immobilized with (a) fibronectin, (b) REDVYpeptide, and (c) KREDVY peptide after 7 days of cell adhesion. Cells onthe platform with chemically bound fibronectin appeared apoptotic andnecrotic (FIGS. 10A and 10D). This might be caused by the competitiveadhesion of multiple cells on a limited surface area of the goldsquare-based cell adhesion regions, hindering individual cell growth andsurvival. This result suggests that large molecule proteins such asfibronectin can result in unpredictable behavior of adhered cells.Because the conformation of fibronectin protein on the gold square-basedcell adhesion regions varies from site to site, the degree ofinteractions of HUVE cells with the binding peptides of the protein andthe number of cells attached on each site can differ from pattern topattern.

No cell necrosis was observed on REDVY and KREDVY modified platformsafter 7 days of cell adhesion. However, cells adhered on thephysically-adsorbed peptide (REDVY) became apoptotic at day 7. Thismight be attributable to the detachment of REDVY peptides from the goldsurface over time, particularly when the cell culture medium wasreplenished, which started apoptosis. A similar observation has beenreported for anchorage-dependent endothelial cells that undergoapoptosis when detached from extracellular matrix substrates. Cellsadhered on the gold based-cell adhesion regions with chemically boundKREDVY remained viable throughout 7 days (i.e., no fluorescencesignals), indicating that the covalently bound KREDVY peptide not onlyfacilitates single-cell adhesion through peptide mediated adhesion, butalso supports prolonged cell attachment and viability.

It has been reported that a decrease in cell adhesion area woulddeleteriously restrict cell spreading and that a 20 μm square islandcoated with fibronectin would lead to cell death. A similar conclusionwas reached in that endothelial cells on 20 μm squares coated withfibronectin underwent apoptosis (FIG. 10D). Surprisingly, the surfacescovalently coupled with short peptide molecules on 20 μm squares canpromote cell spreading and suppress the apoptosis. This observationsuggests that not only the cell adhesion area but also the surfacechemistry plays an important role in cellular viability, and a surfacebound with adhesive short peptides may help retain cellular viabilityfor a prolonged time.

In another aspect, the present invention provides method for using acell-patterning array. In one embodiment, the invention provides amethod for analyzing a plurality of cells immobilized in an array,including:

-   -   (a) subjecting one or more cells individually immobilized in an        array to a stimulus to provide an array comprising individually        treated cells, the array comprising a plurality of cell adhesion        sites isolated on an inert surface, wherein each cell adhesion        site comprises a single cell immobilized thereto by the        interaction of one or more ligands attached to the site and one        or more cell surface receptors of the immobilized cell, and        wherein the inert surface is resistant to cell adhesion;    -   (b) individually addressing one or more of the treated cells to        measure the effect of the stimulus on the treated cells.

The arrays of the invention can be used in a variety of methods thatinvolve interrogating individual cells (or groups of two or three cells)immobilized at each cell adhesion site. The methods of the invention foranalyzing a plurality of cells immobilized in an array can be carriedout using any one of the arrays of the invention described herein. Itwill be appreciated that the methods include arrays having more than onecell immobilized at the cell adhesion site.

In one embodiment, the stimulus is a therapeutic drug compound. Thecell-patterning array of the invention can be used in drug screening.For example, the individual cell on each cell adhesion site in the arraycan be treated with different compounds and the cell response to eachcompound is measured, therefore, achieving high throughput drugscreening by using the cell-patterning array of the invention.Alternatively, the individual cell on each cell adhesion site in thearray can be treated with the same compound at different concentrations,therefore, pharmacological and toxicological data of the compound can beobtained.

In another embodiment, the stimulus is a toxin. In this embodiment, thearray can be used in toxin detection.

In the method, one or more of the treated cells is individuallyaddressed. The cells in the array can be addressed be any means havingthe ability to address and interrogate an individual cell. In oneembodiment, the treated cells can be addressed optically by measuringthe ultraviolet, visible, and infrared adsorption or by measuring thefluorescence or phosphorescence emission. In another embodiment, thetreated cell can be addressed by staining with a biological reportinggroup and then measuring the optical effect of the stained cells. In afurther embodiment, the treated cells can be addressed electrically bymeasuring the capacitance, conductivity, resistivity, or impedance ofthe treated cells immobilized on the array.

The cell-patterning array of the invention can be used in fields such asbiomedical research, diagnostic tools, biosensors, and drug discovery.In one embodiment, the cell-patterning array of the present invention isused in a cell-based biosensor for quick bacterial detection.

An exemplary cell-based biosensor using a cell-patterning array of thepresent invention was prepared as described in Example 2. Each goldmicroelectrode was activated with an alkane thiol self-assemblymonolayer (SAM) and was covalently reacted with a cell adhesive protein(fibronectin) through an N-hydroxysuccinimide (NHS) coupling agent. Thesilicon oxide regions were passivated with methoxypolyethyleneglycol-silane. In this platform, each microelectrode hosts one to threecells, depending on electrode size and cell concentration in culture.

FIGS. 11A-D show the optical DIC images of macrophage patterned on thegold microelectrodes after 21 hours of cell culture for control cellswith no LPS exposure (FIG. 11A) and cells treated with LPS atconcentrations of 0.1 μg/mL (FIG. 11B), 1.0 μg/mL (FIG. 11C), and 10μg/mL (FIG. 11D). The control cells appeared small and round in shape,while LPS-treated cells underwent a morphological change and exhibitedan enlarged, dendritic-like shape. This morphological change was likelyassociated with the synthesis of intracellular peptides and proteinsinduced by LPS.

Cells in an isolated state (e.g., one cell on each microelectrode)generally respond differently to an external stimulus than when they arein a communicating state (e.g., a cluster of cells on a microelectrode).To reveal this difference, macrophage cells were patterned in singlet,doublet, or triplet on gold electrodes of 10 μm×10 μm by culturing cellswith LPS. FIGS. 12A, 12C, and 12E show exemplary optical DIC images ofcell morphology for these cell states, and FIGS. 12B, 12D, and 12F showthe corresponding synchrotron IR spectra of the cells before and afterexposure to LPS at a concentration of 1 μg/mL for 21 hours.

Table 2 lists the characteristic wave numbers acquired from cells of thethree different states before and after exposure to LPS, each averagedover four electrodes of the same state and expressed as mean±S.D. cm⁻¹.Prior to exposure to LPS, cells in the singlet state have acharacteristic amide I peak at 1691±1.2 cm⁻¹, while cells in the doubletand triplet states have the characteristic peaks at 1671±2.5 and1677±3.2 cm⁻¹, respectively.

TABLE 2 Amide I wave number (cm⁻¹) of the single, double and triplecells before and after exposure to LPS (1 μg/mL) Sample Single cellDouble cells Triple cells Control Cells 1691 ± 1.2 1671 ± 2.5 1677 ± 3.2LPS-exposed cells 1661 ± 1.6 1665 ± 2.6 1658 ± 3.4

The difference in amide I characteristic band between the three cellstates, even before cells were exposed to LPS, suggests that thecell-cell interactions affect the IR signatures of cells. The degree ofIR shifts after the cells were exposed to LPS also differedsubstantially among the three cell states with the cells in the singletstate exhibiting the greatest shift. Additionally, the cells in thesinglet state yielded more consistent data than the other two, ascharacterized by its smallest standard deviation. The greateruncertainty in IR shifts produced by the cells in doublet and tripletstates may be attributable to the interactions between cells in the cellclusters, and furthermore, such uncertainty was seen to increase withincreased cell number in the cell cluster.

FIG. 13A shows the synchrotron IR spectra of macrophage cells in singletstate after treated with LPS at different concentrations for 21 hours.FIGS. 13B-13E show exemplary optical images of the cells from which thespectra were acquired. FIG. 13B corresponds to the cell cultured withoutPLS (as control), and FIGS. 13C, 13D, and 13E correspond to the cellscultured with LPS at concentrations of 0.1, 1.0, and 10 μg/mL,respectively. Images in FIGS. 13C-13E show that all the LPS-treatedcells exhibited dendritic morphology and expanded across the electrodeas the LPS concentration increased. The change in IR signature is alsodependent on the LPS concentration, characterized by the shifts of bothamide I and amide II peaks of cell proteins.

The peak of amide I group (predominantly C═O stretching vibration ofamide) shifted from 1691±1.2 cm⁻¹ before cell exposure to LPS, to1676±1.0 cm⁻¹ (10 μg/mL LPS), 1661±1.0 cm⁻¹ (1 μg/mL LPS), and 1659±1.7cm⁻¹ (0.1 μg/mL LPS) post-exposure. These peak shifts in wave number arepresented as main±standard deviation calculated from eight electrodes oftwo substrates for each sample set. The characteristic peaks movedtowards lower wave numbers initially with increased LPS concentration,but to higher wave numbers after reaching a minimum at LPSconcentrations between 0.1 and 1.0 μg/mL. This peak reversion isbelieved to be due to cell death at high LPS concentrations.

To confirm this hypothesis, cellular viability was assessed by stainingcells in singlet state with Annexin V (green for apoptotic cells) andpropidium iodide (red for necrotic cells) after they were exposed to LPSat concentrations of 0.1, 1.0, and 10 μg/mL, respectively. FIGS. 14A and14B show exemplary images of cells treated with LPS at a concentrationof 10 μg/mL, indicating that the cell underwent apoptosis and necrosis.Cellular viability was quantified in terms of ratios of apoptotic andnecrotic cells to the total cells on 238 electrodes on duplicatesubstrates. The result indicated that cells treated with LPS at aconcentration of 10 μg/mL underwent 66.5% apoptosis (positive Annexin Vstaining) and 41.1% necrosis (positive propidium iodide staining),respectively. Control cells and the cells treated with LPS atconcentrations of 0.1 and 1.0 μg/mL showed less than 8% apoptosis and nonecrosis was identified. Images of cells treated with LPS at 0.1 and 1.0μg/mL are not shown in the figure due to absence of statisticallysignificant fluorescence.

These experiments showed a LPS concentration-dependent response ofsingle cells that can be readily detected by FTIR. It is worthwhile tonote that a peak shift of 2-7 cm⁻¹ in wave number has been used toidentify diseased tissue from healthy tissues in multi-cell platforms.Here, a shift in the order of a few tens of wave numbers (e.g., 30 cm⁻¹observed at LPS concentration of 1.0 μg/mL) demonstrated a highsensitivity of this single-cell-based platform. Such sensitivity mayallow for identification of bacterium of very small concentration andsample volume. Furthermore, the degree of bacterium invasion (e.g., thepercent of macrophage cells infected by LPS) can be assessed over alarge number of sensing electrodes, and heterogeneous cellular behaviorcan be investigated with such a microarray of macrophages.

FIG. 15A shows IR spectra acquired by synchrotron-based FTIRmicrospectroscopy from single cells patterned on an array of goldmicroelectrodes before exposure to LPS as well as post-exposure to LPSat a concentration of 1 μg/mL for 3.5 and 21 hours. The optical imagesin FIGS. 15B, 15C, and 15D show the corresponding cell morphology of thesingle macrophage cells on gold electrodes with a size of 100 μm² overthe same time course. The morphology of the LPS-treated cell was seen tochange with LPS exposure time, from a spherical shape to a dendriticshape with increased size over time. The change in IR spectrum over timeduring the LPS exposure is characterized by the continued shifts of bothamide I and amide II peaks from high to low wave numbers and an increasein signal intensity. The IR shifts in amide I spectrum may indicate thechange in protein structure as a result of upregulating various proteinsand peptides involved in the macrophage activation cascade initiated byLPS. It has been reported that LPS induced the synthesis of variouspolypeptides within macrophage cells. Some peptides were short-lived(did not accumulate in LPS-treated cells) and played a regulatory rolewhile others were long-lived (accumulated in LPS-treated cells) andplayed a functional role. Not wanting to be limited by the theory, theIR shift and the intensity increase for cells exposed to LPS for 3.5hours might be due to synthesis of short-lived peptides. The IR peakchange for cells treated with LPS for 21 hours might be attributable tothe presence of long-lived polypeptides. The presence of a single peakfor all the amide I bands in FIG. 15A suggests that proteins withcc-helical secondary structure are dominant. The current experimentsuggests that the variation in wave number in response to LPS invasion,as detected by the single-cell system described here, is adequate foridentification of bacterium in a short period (hours here versus days byconventional bacterial detection methods).

IR signal intensity depends directly on the brightness of IR source andthe size of the electrode that hosts the cell. In a gold-patternedsilicon platform, the maximum signal intensity is obtained when thesynchrotron IR focal point is at the center of the gold electrode andthe noise from the silicon oxide background is minimized. The superiorbrightness of the synchrotron source with a spatial resolution less than10 μm provides high sensitivity for detection of single cells onelectrodes of 100 μm² as shown above. However, a conventional IR thermalsource with an effective beam diameter of about 75 μm requireselectrodes larger than the beam size to reduce the signal loss to thesurrounding area. To study the effect of electrode size on detectionsensitivity and the possible use of conventional FTIR for bacterialdetection with our single-cell system, FTIR spectra from single-cellarrays with electrode sizes of 25, 100, and 400 μm², respectively, wereacquired using both synchrotron and conventional FTIR spectromicroscopy.

FTIR spectra shown in FIGS. 16A and 16B were acquired by synchrotronFTIR. FTIR spectra shown in FIGS. 16C and 16D were acquired byconventional FTIR. The signal intensity was seen to increase withincreased electrode size for both systems. Characteristic peaks of cellmembranes at wave numbers of 2800-3600 cm⁻¹ and cellular proteins at1200-1700 cm⁻¹ are resolved well with the synchrotron source even forthe smallest electrode size (25 μm²) (FIGS. 16A and 16B). Though at asignificantly lower signal intensity, the IR signals acquired with theconventional FTIR are well resolved for the 100 and 400 μm²microelectrodes (FIGS. 16C and 16D). These results indicate that thecurrent single-cell platform can be used with conventional FTIRspectromicroscopy if the electrode surface area is larger than 100 μm².It is noteworthy mentioning that although increasing electrode size willincrease the signal intensity, it also increases the probability ofadhesion of multiple cells on an electrode, rendering single-cellpatterning more difficult. A comparison of the IR spectra acquired fromcells on gold electrodes of different sizes reveals no identifiabledifference in IR signature.

In a related aspect, the invention provides systems for analyzing aplurality of cells immobilized in an array. The system includes one ormore arrays of the invention having one or more cells immobilized on thearrays' cell adhesion sites, and an analytical instrument capable ofindividually addressing and interrogating the immobilized cells.Representative analytical instruments include optical instruments (e.g.,infrared spectrometers, fluorescence spectrometers) and electricalinstruments that measure capacitance, conductance, resistivity,impedance).

In another aspect, the present invention provides arrays having apassivated silicon oxide surface. In this aspect, arrays are providedthat have improved surface characteristics that provide inert surfaceshaving increased stability thereby enhancing their cell resistancecapacity and the lifetime and effectiveness of the arrays.

In one embodiment, the array having a passivated silicon oxide surfacefor guided cell patterning includes a plurality of individuallyimmobilized cells at defined cell adhesion sites isolated on an inertsurface resistant to cell adhesion. For these arrays, the cell adhesionsites are prepared as described above for the arrays of the invention inwhich their silicon substrate surfaces are not treated as describedbelow. In this embodiment, the inert surface is a passivated siliconoxide surface that has been prepared by first depleting native oxidefrom the silicon surface and then oxidizing the native oxide depletedsurface. The inert surface is resistant to cell adhesion by covalentlycoupling polyalkylene oxides to the silicon oxide surface.

In one embodiment, the silicon oxide surface has about 40% to 65% byweight Si, from about 5% to about 20% by weight SiO_(x<2), and fromabout 20% to about 40% by weight SiO₂. In another embodiment, thesilicon oxide surface has from about 50% to 60% by weight Si, from about10% to about 15% by weight SiO_(x<2), and from about 25% to about 35% byweight SiO₂. In a further embodiment, the silicon oxide surface hasabout 58% by weight Si, about 12% by weight SiO_(x<2), and about 30% byweight SiO₂.

These arrays having passivated silicon oxide surfaces are prepared by amethod that includes:

-   -   (a) providing a metal-patterned silicon substrate having an        array of metal surfaces disposed on a silicon surface;    -   (b) exposing the substrate to an oxide etch to remove native        oxide from the silicon surface to provide a native oxide        depleted silicon surface;    -   (c) oxidizing the native oxide depleted silicon surface with an        oxidizing agent to provide a silicon oxide surface; and    -   (d) passivating the silicon oxide surface by covalently coupling        polyalkylene oxide moieties to the silicon oxide surface to        provide a surface resistant to cell adhesion isolating each        metal surface of the metal surface array.

In one embodiment, the method further comprises forming a self-assemblymonolayer on each metal surface to provide an array of monolayersdisposed on the silicon oxide surface. In one embodiment, the methodfurther comprises attaching a plurality of ligands to each self-assemblymonolayer to provide an array of cell adhesion sites. In one embodiment,the method further comprises immobilizing one or more cells at each celladhesion site through the interaction of the ligands and one or morecell surface receptors of the cells. These additional steps can becarried out as described above for the other arrays of the invention.

In the method, exposing the silicon substrate to an oxide etch to removenative oxide from the silicon surface to provide a native oxide depletedsilicon surface and oxidizing the native oxide depleted silicon surfacewith an oxidizing agent to provide a silicon oxide surface, leads to theunique Si/SiO_(x<2)/SiO₂ composition noted above for the array having apassivated silicon oxide surface.

Native oxide on the silicon surface can be removed by any usefultechnique that is effective in removing the native oxide to provide anative oxide depleted silicon surface. Representative oxide etchesinclude a mixture of hydrogen fluoride (HF) and ammonium fluoride(NH₄F), Piranha (HF/NH₄F 5:1 v/v), and NANOSTRIP (H₂SO₅).

The native oxide depleted silicon surface can be oxidized by anysuitable oxidizing agent to afford a silicon oxide surface.Representative oxidizing agents include dry oxygen, ozone, hydrogenperoxide, and chromic acid.

In one embodiment, the native oxide depleted silicon surface is oxidizedwith dry oxygen at an elevated temperature to provide a silicon oxidesurface with a dry thermally grown oxide layer. In one embodiment, thenative oxide depleted silicon surface is oxidized with dry oxygen atfrom about 300° C. to about 500° C. for from about 5 to about 24 hoursto provide a silicon oxide surface. A representative silicon oxidesurface with dry thermally grown oxide can be obtained by exposing thesurface to buffered oxide etch (HF/NH₄F 5:1 v/v) for 60 seconds toremove native oxide on the surface, rinsing with deionized water toremove the native oxide on silicon regions, and flushing the treatedsurface with a dry oxygen flow for 6 hours at 400° C. to provide anoxide surface with a 60 Å oxide layer on the surface.

In one embodiment, the native oxide depleted silicon surface is oxidizedwith wet oxygen at an elevated temperature to provide a silicon oxidesurface with a wet thermally grown oxide layer. A representative siliconsurface with wet thermally grown oxide can be obtained by exposing thesurface to buffered oxide etch (HF/NH₄F 5:1 v/v) for 60 seconds, rinsingthe surface with deionized water to remove the native oxide to provide atreated surface, and flushing the treated surface with a wet oxygen flowat 850° C. to provide an oxide surface with a 1,000 Å oxide layer on thesurface.

The thickness of the oxide layer can vary according to oxidationcondition. In general, the thickness of the oxide layer can be fromabout 30 Å to about 1,500 Å depending upon the condition of theoxidation reaction. In one embodiment, the thickness of the oxide layeris about 60 Å. In another embodiment, the thickness of the oxide layeris about 1,000 Å. In one embodiment, the silicon oxide surface comprisesan oxide layer having a thickness of from about 50 Å to about 1500 Å. Inanother embodiment, the silicon oxide surface comprises an oxide layerhaving a thickness of about 60 Å. In one embodiment, the silicon oxidesurface comprises an oxide layer having a thickness of about 1000 Å.

Several arrays for cell-patterning have been prepared and theircell-patterning capabilities were compared, as described in Example 3.Three types of surfaces that contained native oxide, dry thermally grownoxide, and wet thermally grown oxide, respectively, were produced as abasis for the inert surface. As used herein, “silicon surface withnative oxide” refers to a silicon surface without any chemicalmodification. The cell-patterning platform based on a silicon surfacewith native oxide was used as a comparison for evaluating theeffectiveness of the cell-patterning platforms of the present inventionhaving a passivated silicon oxide surface.

Silicon substrates were cleaned with piranha (hydrogen peroxide/sulfuricacid 2:5 v/v) at 120° C. for 10 minutes, dipped in HF and rinsed with DIwater thoroughly. A layer of positive photoresist was then coated on thesurface and an array of gold square was patterned on the silicon oxidesubstrate by conventional microfabrication. Specifically, a layer oftitanium (Ti) was deposited onto photoresist-developed substrates. Agold film was subsequently deposited onto the titanium. The photoresistwas dissolved and the remaining metal film was lifted off.

The surface with native oxide was formed as a result of exposure of thesubstrates to the air. The surface with dry oxide was created byadditionally exposing the surface to buffered oxide etch (HF/NH₄F 5:1v/v) for 60 s and rinsing with DI water to remove the native oxide onsilicon regions, followed by flushing with a dry oxygen flow for 6 h at400° C., yielding a 60 Å oxide layer on the silicon regions. The surfacewith wet oxide was prepared following the same procedure except that thesubstrates were placed under a wet oxygen flow at 850° C., yielding a1000 Å oxide layer on the silicon regions.

The gold-patterned substrates were then modified. The silicon regionswith native, dry, or wet oxide were reacted with a low molecular weightM-PEG-silane (molecular weight=460-590 Dalton) to form inert (i.e.,non-cell adhesive) regions. The gold regions were first reacted withCOOH-terminated alkyl thiols to form a COOH-terminated SAM containing aplurality of COOH groups, followed by covalently bonding proteins to atleast a portion of COOH groups to form cell adhesive regions.

High-resolution XPS spectra were acquired on the solid siliconsubstrates before and after surface modification with methoxy-PEG-silane(M-PEG). The results were shown in FIGS. 2A-2H. The native oxidesubstrate showed the expected binding energy of elemental silicon (about99.7 eV) and SiO₂ (about 103.5 eV). The dry oxide surface showed asimilar spectrum as that of native oxide but with an asymmetric peak atabout 102 eV, indicating the presence of a different silicon oxide statethan those present on the native and wet oxide surfaces. This additionaloxide state corresponds to silicon oxidation states (SiO_(x<2)) otherthan SiO₂. The wet silicon oxide surface had only a SiO₂ peak in its XPSspectrum indicating that the silicon background was completely coveredby silicon dioxide.

Survey spectra indicated an increase in the carbon content and adecrease in the silicon content on all of the PEG-modified surfacescompared to that of their unmodified counterparts. More oxygen wasobserved on both native and dry oxide after PEG modification, as opposedto wet oxide surfaces. The decreased amount of oxygen on the wet oxidewas due to the fact that the wet oxide surface was mainly composed ofsilicon dioxide before PEG modification.

Table 3 represents the high-resolution XPS analyses of the Si_(2p)components on the oxide surfaces and the C_(1s) components on thePEG-modified surfaces. The spectra of high-resolution C_(1s) for allPEG-modified surfaces appeared to be similar, and the amounts of PEG onthem were about the same. Further elaborations on the PEG componentanalysis are presented in the following sections.

TABLE 3 High resolution XPS Si_(2p) and C_(ls) analyses^(a) (A) %composition (B) % composition sample Si SiO_(x<2) SiO₂ sample C—H C—OC═O native 69.64 30.36 native 15.1 80.6 4.3 oxide oxide dry oxide 58.4511.85 29.7 dry oxide 14.8 82 3.2 wet oxide 100 wet oxide 15 79.7 5.3^(a)Spectra were taken at a 55° takeoff angle from (A) silicon oxidesubstrates and (B) PEG-modified silicon oxide substrates. Thepercentages are atomic percents of each type of surface atom calculatedfrom survey spectra scan (FIGS. 2A-2H).

Water contact angles on all three types of silicon surfaces weremeasured after the surfaces were modified with PEG. The contact anglevalues of the three surfaces are 31.6±1.34° for native oxide; 29.8±0.83°for dry oxide; 30.4±2.4° for wet oxide.

Despite the apparent differences in the nature of silicon oxide layersgrown on the three surfaces before M-PEG-silane modification, both thecontact angle measurements and XPS data indicated that about the sameamount of PEG was coated on all three surfaces.

Protein adsorption on the gold-patterned surfaces was visualized withfluorescence microscopy on unmodified (clean Au/SiO₂) surfaces and onthe cell-patterning platform having the cell-adhesive regions comprisingalkyl SAM on gold and the inert regions with PEG on silicon oxide asshow in FIG. 1B. The platform was exposed to fibronectin-Cy3 conjugateimmediately following the surface modification. FIGS. 3A-F showfluorescent images of the unmodified (left panel) and modified (rightpanel) surfaces with the silicon background immobilized with nativeoxide (FIGS. 3A and 3B), wet oxide (FIGS. 3C and 3D), and dry oxide(FIGS. 3E and 3F).

For the unmodified surfaces, proteins were randomly adsorbed over boththe Au and silicon oxide regions (FIGS. 3A, 3C, and 3E), andparticularly, the surface with wet oxide was heavily covered withproteins with no electrodes visible. The results reveal the influence ofthe silicon oxide state on protein adsorption. After surfacemodification, the protein was selectively adsorbed on gold electrodes ofall three surfaces (FIGS. 3B, 3D, and 3F). However, a slightnonspecifically adsorbed protein was seen on the silicon region of thewet oxide surface (FIG. 3D). This phenomenon can be attributed to thestrong affinity of the wet oxide for protein adsorption (FIG. 3C). Thishigh affinity makes the wet oxide surface more susceptible to thedefects presented in the PEG coating in preventing protein adsorptionthan the other surfaces, resulting in a high local protein adsorption(and subsequent cell adhesion when exposed to cells) on defected sites.

The PEG-modified dry oxide surface showed high protein resistance.Quantitative measurements of fluorescence intensities of the siliconsurfaces yielded the following: wet oxide, 60; native oxide, 17; and dryoxide, 11, from the unmodified surfaces. This indicates that the dryoxide layer on the silicon region is the least biofouling layer byitself, and thus, the order of possible biofouling appeared to be: dryoxide<native oxide<wet oxide.

Cell selectivity was studied by culturing murine macrophage cells on thecell-patterning platform and monitored for up to 10 days, which is atypical time period for practical applications of cell-based biosensors.Cell selectivity was defined as selective confinement of cells to thedesignated regions—the gold-based cell-adhesive regions in the presentinvention. Macrophage was used as the model cell line for cellselectivity study because of its important physiological functions inthe human body. For example, they have potentials for use as cellulardelivery vehicles for gene therapy of diseased tissues and are animportant source of mitogenic growth factors and proangiogenic cytokinesin wound healing. Selective suppression of macrophage activation is alsoa possible approach to diminishing local inflammation. Combined with inkjet or other analyte-positioning techniques, surfaces patterned withmacrophages may be used as sensing arrays for rapid detection of avariety of external stimuli and screening of drugs.

DIC reflective images were acquired after 2 days of cell culture and upto 10 days because all of the surfaces had formed a uniform and highlyselective cell pattern up to 1 day during which time no apparentdifference on cell morphology was observed. These results are shown inFIGS. 4A-4I. Images were acquired on day 3, 7, and 10 (presentedhorizontally from left to right over time). FIGS. 4A-4C, 4D-4F, and4G-4I are the substrates with native oxide, wet oxide, and dry oxide,respectively.

It is noted that the cell selectivity over time differed dramaticallyamong the three patterned platforms. Cells started to migrate to theinert regions on the native oxide surface on day 3 (FIG. 4A), and thepatterned surface completely lost cell selectivity on day 10 (FIG. 4C).The cell-patterning platform with wet oxide-based inert regions (FIGS.4D-4F) has much better cell selectivity than the platform with nativeoxide-based inert region. The platform with dry oxide-based inertregions shows very high cell selectivity through the duration of thestudy (10 days) as shown in FIGS. 4G-4I. Dry oxide has an additionaladvantage of being a better insulator, which would enhance theperformance of cell-based biosensors by increasing the electricalsignal-to-noise ratio.

Not wanting to be bound by the theory, it is noted that the prolongedcell selectivity on the platform with dry oxide-based inert regionsmight be related to the intermediate oxidation state for the silicon(Table 3 A). This is a more chemically reactive state than a fullystable SiO₂ and may result in different types of reactions. It has beenreported that methyl trimethoxy silanes react directly withdehydroxylated silica surfaces to form stable, chemically boundalkylsiloxanes and alkoxides at 300 K and that model silane couplingagents can react directly with the highly strained siloxane bondspresent on dehydroxylated silica without the involvement of surfacehydroxyl groups. In addition, it has been shown that trimethoxy silanesthat are strongly bonded on surfaces at 330 K are extremely thermallystable. The fact that the PEG used for this study was a trimethoxysilane-PEG and was reacted with the surface at about 333 K may justifythe observed behavior of the dry oxide surface. Thus, the interaction ofPEG with the active state of silicon oxide might have contributed toformation of a more stable PEG coating on the silicon substrate.

To validate this theory, the stabilities of the PEG coatings on threemodel surfaces under cell culture condition were assayed usingserum-containing medium. PEG-modified surfaces were incubated in themedium at 37° C. and 5% humidity, and XPS analysis was performed on thesamples on day 0, 3, 7, and 10. The results are shown in Table 4.

TABLE 4 High resolution XPS C1s analysis of PEG-modified substrates^(a)% composition day 0 day 3 day 7 day 10 C—H C—O C═O C—H C—O O—C═O C—H C—OO—C═O C—H C—O O—C═O sample ~285 ~287 ~288 ~285 ~287 ~289 ~285 ~287 ~289~285 ~287 ~289 PEG-native 15.1 80.6 4.3 72.0 22.6 5.4 72.7 22.3 5 70.821.9 7.3 PEG-dry 14.8 82 3.2 19.1 79.0 1.9 20.1 78.1 1.8 37 60.7 2.3PEG-wet 15 79.7 5.3 17.9 76.9 5.2 22.3 72 5.7 49.8 46 4.2 ^(a)Spectrawere taken at 55° takeoff angle, after the substrates were incubated incell culture medium for 3, 7, and 10 days.

Most noticeable is the appearance of the O—C═O peak at about 289 eV onall the platforms after 3 days of cell culture. This can be attributedto the partial oxidation of PEG in the cell culture medium. The amountof O—C═O was maximum on the native and wet oxide-based inert regions andminimum on the dry oxide-based inert regions through the period ofstudy. The change in this peak provides an indication of the PEGoxidation level over time but does not rule out the possibility of PEGdecomposition or degradation. PEG degradation can be estimated by thechange in the amount of C—O over time. In this context, the degradationrate is defined as ΔCO_(t)/CO₀=(CO₀−Co_(t))/CO₀, where CO₀ and Co_(t)represent the amount of C—O initially and at time t, respectively. Ahigh ΔCO/CO₀ value corresponds to a high degradation rate. The resultscalculated from data in Table 4 are shown in Table 5.

TABLE 5 Time-dependent degradation of the PEG coatings in cell culturemedium at 37° C. and 5% humidity. degradation rate (CO₀ − CO_(t))/CO₀sample PEG-native PEG-dry PEG-wet day 3 0.719 0.036 0.035 day 7 0.7230.047 0.096 day 10 0.728 0.259 0.422

PEG on the native oxide-based inert region showed a considerabledegradation on day 3 (ΔCO/CO₀=0.719). The PEG degradation proceeded muchslower on the dry oxide and wet oxide-based inert regions (ΔCO/CO₀=0.036and 0.035, respectively). PEG on the both types of inert regionsremained stable up to 7 days and was partially degraded afterward. PEGon dry oxide had the least degradation for up to 10 days. When thisresult is compared with the cell-patterning images shown in FIGS. 4A-I,a marked consistency is seen: the cell selectivity over time is directlycorrelated to PEG degradability.

Not wanting to be limited by the theory, it is believed that a stablePEG coating is essential to achieving improved cell selectivity and thatthe prolonged PEG integrity in cell cultural medium may be related tothe presence of oxide states other than silicon substrates. Such anoxide surface might have served a dual function purpose: a relativelylow affinity to proteins that suppress the protein adsorption in asurface with favorable chemistry on which a stable PEG coating can bedeveloped. The silicon substrate with trioxide achieves a more stablePEG coating than the silicon surfaces with native or wet oxide that aretypically used in development of bio-MEMS devices.

Thus, maintaining PEG integrity over time, particularly in cell culturemedium, as far as ligand-mediated cell patterning is concerned, is a keyto the success of prolonged cell selectivity and bio stability. This wasachieved by the present invention by engineering the silicon surface tochange its native oxidation state, on which a more stable PEG coatingcan be obtained.

The following examples are provided for the purpose of illustrating, notlimiting, the invention.

EXAMPLES

Materials. The following materials and chemicals were used as received:NANOSTRIP 2X (Cyantek, Fremont, Calif.),2-[methoxy(polyethyleneoxy)propyl]trimethoxysilane (M_(w)=460-590 Da)(Gelest, Morrisville, Pa.), Cy3 monoreactive NHS ester (AmershamBiosciences, Sweden), RPMI-1640 (ATCC, Manassas, Va.), heat-inactivatedfetal bovine serum (Invitrogen, Carlsbad, Calif.),penicillin-streptomycin (Gibco, Carlsbad, Calif.), 11-mercaptoundecanoicacid 95% (11-MUA), 3-mercaptopropionic acid 99% (3-MPA),N-hydroxysuccinimide 97% (NHS),1-ethyl-3-(3-(dimethylamino)-propyl)carbodiimide (EDAC), fibronectinprotein, trypsin-EDTA, sigmacote, and glutaraldehyde (Sigma-Aldrich,Milwaukee, Wis.), paraformaldehyde, 4′,6-diamidino-2-phenyindole (DAPI),anti vinculin-FITC (Sigma, St. Louis, Mo.), silicon wafers of (100)orientation (Wafernet, Calif.), lipopolysaccharide, LPS (E.-coli0111:B4, endotoxin unit: 500,000) (Sigma, Milwaukee, Wis.).

ALEXA FLUOR 594 phalloidin and VYBRANT Apoptosis Assay Kit #2 wereobtained from Molecular Probes (Eugene, Oreg.). REDVY (518.3 Dalton) andKREDVY (806.1 Dalton) were purchased from Synpep (Dublin, Calif.). Humanumbilical cord vein endothelial (HUVE) cells and cell culture suppliesincluding EGM-2, HEPES-buffered saline, trypsin EDTA, and trypsinneutralizing solution were purchased from Clontics (Walkersville, Md.).RAW264.7 cells (murine monocyte/macrophage) were purchased from AmericanType Culture Collection (Manassas, Va.). The following cell culturereagents were purchased from Gibco (Carlsbad, Calif.): Trypan Blue,Fetal Bovine Serum, HBSS (Hanks Balanced Salt Solution), DMEM(Dulbecco's Modified Eagle's Medium with 4 mM L-glutamine adjusted tocontain 1.5 g/L sodium bicarbonate and 4.5 g/L glucose).

All the solvents including toluene, triethylamine, and dimethylformamidewere purchased from Aldrich (Milwaukee, Wis.). Absolute ethanol wasdeoxygenated by dry N₂ before use.

Example 1 Representative Cell-Patterning Arrays with Cell-AdhesionPeptides Attached to Cell-Adhesion Sites

In this example, the preparation of a dry oxidized, native oxidedepleted silicon surface useful in making representative cell-patterningarray of the invention is described.

Substrate Preparation. Four-inch p-type silicon substrates with (100)orientation were cleaned with piranha (hydrogen peroxide/sulfuric acid2:5 v/v) at 120° C. for 10 minutes, dipped in HF, and rinsed with DIwater thoroughly. A 1.1 μm layer of positive photoresist was then coatedon the surface, and an array of 20 μm×20 μm gold squares (electrodes)spaced 60 μm apart was patterned on silicon oxide substrates byconventional microfabrication as follows. A 10 nm layer of titanium (Ti)was then deposited onto the photoresist-developed substrates at adeposition rate of 0.3 Å/s. A gold film of 100 nm in thickness wassubsequently deposited onto the Ti at a deposition rate of 5 Å/s. Thephotoresist was dissolved in acetone and the remaining metal film waslifted off. After lift-off, the surfaces were exposed to buffered oxideetch (HF/NH₄F 5:1 v/v) for 60 seconds and rinsed with DI water to removethe native oxide on silicon regions, followed by oxidation under a dryoxygen flow for 6 hours at 400° C., yielding a 60 Å oxide layer on thesilicon regions. The gold-patterned silicon wafers were cut into 8 mm×8mm slides. To minimize surface contaminants and scratches, the siliconwafers were coated with a 2 μm layer of photoresist on their polishedsides before cutting.

Surface Modification. The protective photoresist layer on gold-patternedsilicon oxide substrates was removed by sonication in acetone, then inethanol, and finally in DI water. The substrates were then placed inNANOSTRIP 2X solution (H₂SO₅) at room temperature for 20 minutes anddried under nitrogen, which resulted in a hydroxyl layer on the siliconoxide surfaces. The gold-patterned silicon oxide substrates were reactedwith a 20 mM solution of alkane thiols of 11-mercaptoundecanoic acid(MUA) and 3-mercaptopropionic acid (MPA) (1:10 v/v) for 16 hours tocreate a self-assembly monolayer (SAM) on gold squares. The substrateswere then exposed to PEG solution containing 3 mM methoxy-PEG-silane(M-PEG-silane) and 1% triethylamine as a catalyst in deoxygenatedtoluene to passivate silicon oxide. The PEG reaction proceeded at 60° C.for 18 hours in nitrogen-filled flasks that were pre-treated withSigmacote to minimize the side reaction of PEG with the flasks. ThePEG-treated surfaces were cleaned by sonication in toluene and ethanolfor 5 minutes each, followed by a rinse with DI water and drying undernitrogen. The substrates with a SAM of alkane thiol on gold andmethoxy-PEG-silane on silicon oxide were then immersed in an aqueoussolution of 150 mM EDAC and 30 mM N-hydroxysuccinimide (NHS) for 30minutes to attach the NHS group to the —COOH terminus of the SAMs. Thesubstrates with NHS on gold and PEG on silicon oxide were sterilizedwith 70% ethanol for 15 minutes, and exposed to either fibronectinprotein, REDVY, or KREDVY peptide in a phosphate buffer solution (PBVS,pH=8.2) at a concentration of 0.1 mg/ml. The reaction continued at roomtemperature for 1 hour. To remove loosely bound moieties from thesurface after each step of surface modification, the substrate wasrinsed with its original solvent and DI water, respectively.

Surface Characterization by FTIR. Surfaces coated with fibronectinprotein or peptides were characterized using a Nicolet Magna 760 fouriertransform infrared (FTIR) spectroscope equipped with an FT-85 grazingangle sample compartment. FTIR absorption spectra of 750 scans wereacquired at a resolution of 8 cm⁻¹. The system was purged with dry airfor 1 hour before each data collection to remove water vapor in thesample compartment. Spectra analysis was performed using standardNicolet and Origin software. The grazing angle FTIR adsorption spectraof surfaces modified with various coatings are shown in FIG. 6.

Cell Culture and Adhesion. HUVE cells were cultured in EGM-2 mediumsupplemented with bovine brain extract (BBE), hydrocortisone, hFGF-B,VEGF, R3-IGF-1, ascorbic acid, heparin, FBS, hEGF, and GA-1000. Thefinal serum concentration was 2%. Cells were seeded on culture flasks atpassage 1 and the medium was changed after 24 hours. At 70% confluency,cells were subcultured as follows. After aspiration from culture flasks,cells were rinsed with 2-3 ml of HEPES-BSS buffer solution for 3 times,followed by incubation with 2 ml of trypsin/EDTA solution. Thetrypsinization process continued until about 90% of the cells werecollected. After cells were released, the trypsin was neutralized in theflask with 4 ml of TNS, and the detached cells were transferred to a15-ml sterile centrifuge tube. The harvested cells were centrifuged at220 g for 5 minutes. Cells were diluted in growth medium. Cells at aconcentration of 1.5×10⁵ cells/ml were seeded on the substrates. Thesubstrates were incubated for 18 hours before fixation with a mixture of2% glutaraldehyde and paraformaldehyde in phosphate buffer solution foroptical microscopy and with 4% paraformaldehyde for fluorescencemicroscopy. The optical micrographs of the cells are shown in FIGS.7A-7C. The fluorescent images of cells are shown in FIGS. 9A-9D.

Cell Staining. Cells adhered on the substrates were fixed,permeabilized, and stained with ALEXA FLUOR 594 phalloidin dye forF-actin staining (red) and immunostained with monoclonalanti-vinculin-FITC (green) followed by cellular staining with6-diamidino-2-phenyindole (DAPI, blue). Before fixation, the substrateswere washed with PBS to remove cell debris and loosely attached cells.Cells were fixed with 4% paraformaldehyde in PBS for 30 minutes at roomtemperature and permeabilized by treating cells with Triton X100 (0.1%in PBS) for 10 minutes. Following three washes with PBS the samples wereincubated for 30 minutes with a 1× blocking buffer solution (5% (w/v) ofnonfat dry milk in PBS containing 0.1% Tween-20) for backgroundpassivation. The actual staining was done in two steps. First, theprimary antibody against vinculin (anti vinculin-FITC) was diluted inblocking buffer following the manufacturer's recommendations and treatedwith cells over night in dark at 4° C. Next, the samples were washedthree times with blocking buffer before cells were exposed to aphalloidin-ALEXA FLUOR 546 for 1 hour at RT. The samples were thenwashed with PBS three times and blown dry with air. A final treatmentwith gold anti-fade solution containing 6-diamidino-2-phenyindole (DAPI)stained cell nuclei and preserved the fluorescence of the samples forconfocal microscopy. The fluorescent confocal images of cells afterstaining are shown in FIGS. 8A-8D.

Cell Viability Assay: Apoptosis and Necrosis. VYBRANT apoptosis assayallows for the simultaneous visualization of viable, necrotic, andapoptotic cells on substrates. Necrosis results from direct cell damage;apoptosis is genetically-programmed cell death in which cellseffectively commit suicide. Green fluorescently labeled Annexin Vprotein (in the presence of calcium) specifically binds to thephosphatidylserine protein on membranes of apoptotic cells. Propidiumiodide does not penetrate to either live or apoptotic cells, but rather,stains nuclei of necrotic cells in red. Cell-patterned substrates werewashed twice with cold PBS and placed in 500 μL of Annexin V bindingbuffer (10 mM HEPES, 140 mM NaCl, 2.5 mM CaCl₂, pH 7.4). The substrateswere incubated with 100 μL of Annexin V and 2 μL of PI solution for 15minutes at room temperature, washed twice with binding buffer, andimaged with a fluorescence microscope. The fluorescence images showingthe apoptotic cells and necrotic cells are shown in FIGS. 10A-10F.

Example 2 Detection of Bacterial Infection using a RepresentativeCell-Patterning Array

In this example, a method for using a representative array of theinvention for detecting bacterial infection is described.

Substrate Preparation. Four inch p-type silicon substrates of (100)orientation were cleaned with piranha (hydrogen peroxide/sulfuric acid2:5, v/v) at 120° C. for 10 minutes, dipped in HF, and thoroughly rinsedwith DI water. A layer (1.1 μm) of positive photoresist was then coatedon the surface, and patterns were formed on the substrate upon exposureto ultraviolet light through a mask with square patterns of threedifferent sizes (25, 100, and 400 μm²). A titanium (Ti) layer (10 nm)was then deposited on the photoresist-developed substrates at adeposition rate of 0.3 Å/s. A gold film of 100 nm thickness wassubsequently deposited on the Ti at a deposition rate of 5 Å/s. Thephotoresist was dissolved in acetone and the remaining metal film waslifted off. After lift off, the surface was exposed to buffered oxideetch (HF/NH₄F 5:1, v/v) for 60 seconds and rinsed with DI water toremove native oxide on silicon before oxidation. The surface oxidationwas performed under a dry oxygen flow for 6 hours at 400° C. Thegold-patterned silicon oxide substrates were then cut into slides of 8mm×8 mm. To prevent surface contamination and scratches, the siliconoxide wafers were coated with a 2 μm layer of photoresist on theirpolished sides before cutting.

Surface Modification. The surface was modified following a previouslyestablished procedure with minor modifications. Veiseh, M. et al.,Langmuir 18, 6671-6678, 2002; and Lan, S. et al., Biosens. Bioelectron.20, 1697-1708, 2005. The protective photoresist layer on gold-patternedsilicon substrates was removed by sonication for 10 minutes in acetone,2 minutes in ethanol, and 2 minutes in DI water. The substrates werethen placed in NANOSTRIP 2X solution (H₂SO₅) at room temperature for 20minutes, and dried under nitrogen, resulting in a hydroxyl layer on thesilicon oxide surface.

The gold electrodes on the substrate were first reacted with a 20 mMmixture of alkane thiols of 11-merecaptoundecanoic acid (MUA) and3-mercaptopropionic acid (MPA) (1:10, v/v) for 16 hours to create aself-assembly monolayer (SAM). The silicon oxide background waspassivated with polyethylene glycol (PEG). The PEG solution was preparedin a nitrogen-filled reaction flask by adding 3 mM M-PEG-silane indeoxygenated toluene containing 1% triethylamine as catalyst. TheNANOSTRIP-treated substrate was then placed in a separatenitrogen-filled flask that was rendered hydrophobic with Sigmacote tominimize the side reaction of PEG with the flask. The PEG reactionproceeded under nitrogen at 60° C. for 18 hours. Physically adsorbedmoieties were removed from the PEG-treated surface by sonication intoluene and ethanol for 5 minutes each, followed by rinsing with DIwater and drying under nitrogen. The substrate with alkane thiol SAM ongold and M-PEG-silane on the silicon oxide background was immersed in anaqueous solution of 150 mM EDAC and 30 mM N-hydroxysuccinimide (NHS) for30 minutes to attach the NHS group to the —COOH terminus of SAM. Thesubstrate with NHS on gold and PEG on silicon oxide was sterilized with70% ethanol for 15 minutes and exposed to fibronectin protein at aconcentration of 0.05 mg/mL in a phosphate buffer solution (PBS) of pH8.2 at room temperature for 45 minutes. To remove loosely bound moietiesfrom the surface after each step of the surface modification, thesubstrate was rinsed with the original solvent and then DI water.

Cell Culture. RAW264.7 of passage less than 10 was cultured at 37° C. ina 5% CO₂-humidified incubator and grown in DMEM medium supplemented with10% (v/v) heat-inactivated FBS, 4 mM L-glutamine, 1.5 g/L sodiumbicarbonate, 4.5 g/L glucose, 100 units/mL penicillin, and 100 g/mLstreptomycin. Cells were subcultured by a cell scrapper once a week.Culture conditions were the same for both LPS treated and control cellson surfaces. LPS treatments were performed using a stock solution oflipopolysaccharide (500,000 endotoxin units/mg) from E. coli 0111:B4 inHBSS at 1 mg/mL. RAW264.7 cells at a concentration of 2.5×10⁵ cells/mLin DMEM medium were exposed to LPS at doses of 0.1, 1, or 10 μg/mL, and0.5 mL of solutions were incubated with the surfaces for up to 21 hoursunder sterile condition to avoid contamination.

Cell Viability Assay. After cell culture, both LPS treated andun-treated (control) cell-patterned substrates were washed twice withPBS and placed in 500 μL of Annexin V binding buffer (10 mM HEPES, 140mM NaCl, 2.5 mM CaCl₂, pH 7.4). The substrates were incubated with amixture of 100 μl of Annexin V and 2 μl of propidium iodide solutionsfor 15 minutes at room temperature, washed twice with the bindingbuffer, and visualized with a fluorescence microscope. The greenfluorescently labeled Annexin V protein (in the presence of calcium)specifically binds to the phosphatidylserine protein on membranes ofapoptotic cells. Propidium iodide does not penetrate either live orapoptotic cells, but stains nuclei of necrotic cells in red.

Differential Interference Contrast (DIC) Reflectance Microscopy.Cell-cultured surfaces were examined with a differential interferencecontrast (DIC) reflectance microscope (Nikon E800 Upright Microscope,NY, N.Y.) equipped with DIC-20× (N.A. 0.46) and DIC-50× (N.A. 0.8)objectives. Images were acquired with a Coolsnap camera (seriesA99G81021, Roper Scientific Inc., AZ, USA) attached to the microscopeand a computer. The optical DIC images of cells after exposing to LPS atvaried concentrations for 21 hours are shown in FIGS. 11A-11D and FIGS.13B-13E. FIGS. 12A, 12C, and 12E are optical DIC images of electrodeswith a single cell, double cells, and triple cells, respectively. Theoptical DIC images of cells stained with Annexin V and propidium iodideare shown in FIGS. 14A and 14B.

FTIR Spectromicroscopy of Cells on Patterned Substrates. IR spectra andoptical reflectance DIC images were acquired from cells on the patternedsubstrates with single or a group of macrophage cells on each electrodeboth before and after cellular exposure to LPS. Synchrotron FTIR spectrawere acquired from cell-patterned surfaces with a Nicolet Magna 760 FTIRbench and a Nicolet Nic-Plan™ IR microscope equipped with acomputer-controlled x-y-z sample stage (via Nicolet Atlμs™ and OMNICsoftware) and an MCT-A detector at Beamline 1.4.3 of the Advanced LightSource (ALS) in Lawrence Berkeley National Laboratory, Berkeley. C A.Martin, M. C. and McKinney, R. W., Proceed. Mater. Res. Soc. 524, 11-15,1998; C A. Martin, M. C. and McKinney, R. W., Proceedings of the LowEnergy Electrodynamics in Solids, '99 Conference, Pécs, Hungary, SpecialIssue of Ferroelectrics, 249, 1-10, 2001. In order to align the incidentIR beam onto the substrate, an IR map (with 2-10 μm step size in x-yplane) was acquired around a gold electrode for a full IR range of400-10,000 wave numbers. Under this condition, the whole spectrumappeared as a broad peak and an intensity profile was given for themapped region. The x, y positions were adjusted so that the highestintensity region of the beam was aligned with the center of the goldelectrode. The samples were measured at wave numbers of 650-10,000 cm⁻¹using an XT-KBr beamsplitter and an MCT detector. The synchrotroninfrared light is focused to a diffraction-limited spot size with awavelength-dependent diameter of approximately 3-10 μm across the mid-IRrange of interest. Carr, G. L. Review of Scientific Instruments 72,1613-1619, 2001; and Dumas, P., et al., Faraday Discussions 126,289-302, 2004. An on-stage temperature controlled mini incubator wasused to maintain a proper environment for cellular analysis. Prior toinfrared analysis, dead and loosely bound cells were removed from thesubstrate by three PBS washes (to eliminate the possible interference ofdead and loosely bound cells to the real-time signals generated by livecells), the cell culture medium was replaced with fresh sterile medium,and the substrate covered with a layer of the medium was transferred tothe mini incubator. The spectra were acquired in less than 10 minutesfollowing the sample transfer to ensure cell viability and to minimizepossible interference from environmental changes. Synchrotron FTIRspectra of 128 scans at a resolution of 8 cm⁻¹ were acquired fromindividual electrodes patterned with cells. Background signals werecollected from the silicon oxide surface of the same substrate rightbefore the data collection. Images of 75 electrodes were captured andsignals from four electrodes hosting cells of similar morphologies werecollected and averaged for each type of LPS treatment. All spectra werebaseline-corrected and normalized to account for the continuous decay ofthe synchrotron beam in the storage ring. An appropriately scaled watervapor spectrum was subtracted from the spectra of cells. The spectraobtained with conventional FTIR were acquired from cell-patternedsurfaces using a Thermo-Electron Nexus 870 bench and a Thermo-ElectronContinuum infrared microscope with an MCT-A detector at Beamline 1.4.4of the ALS under the same conditions set for the synchrotronmeasurements, except that an aperture size of 90 μm×90 μm were employedto maximize the signal intensity. The real-time synchrotron IR spectraof cells before and after treatment of LPS are shown in FIGS. 12B, 12D,and 12F. The real-time synchrotron FTIR spectra of cells treated withvaried concentrations of LPS are shown in FIG. 13A. FIG. 15A shows thereal-time synchrotron IR spectrum of a single cell response to LPS overtime.

Example 3 Comparison of Cell-Patterning Arrays Based on Native SiliconSurface, Wet Oxide Surface, and Dry Oxide Surfaces

In this example, the preparations of cell-patterning arrays based onnative silicon surface, wet oxidized native oxide depleted siliconsurface, and dry oxidized native oxide depleted silicon surface aredescribed. The cellular viability and stability of these arrays arecompared.

Substrate Preparation. Four inch p-type silicon substrates of (100)orientation were cleaned with piranha (hydrogen peroxide/sulfuric acid2:5 v/v) at 120° C. for 10 minutes, dipped in HF, and rinsed with DIwater thoroughly. A layer of positive photoresist (1.1 μm) was thencoated on the surface, and an array of squares (20 μm×20 μm) waspatterned on the substrate upon exposure to UV light through a mask. Athin layer of titanium (Ti) of 10 nm in thickness was then depositedonto the photoresist-developed substrate at a deposition rate of 0.3Å/s. Gold films of 100 nm in thickness were subsequently deposited onthe Ti at a deposition rate of 5 Å/s. The photoresist was dissolved inacetone, and the remaining metal films were lifted off. The surface withnative oxide was formed as a result of exposure of the substrates to theair. The surface with dry oxide was created by additionally exposing thesurface to buffered oxide etch (HF/NH₄F 5:1 v/v) for 60 seconds andrinsing with DI water to remove the native oxide on silicon regions,followed by flushing with a dry oxygen flow for 6 hours at 400° C.,yielding a 60 Å oxide layer on the silicon regions. The surface with wetoxide was prepared following the same procedure except that thesubstrates were placed under a wet oxygen flow at 850° C., yielding a1000 Å oxide layer on the silicon regions. The gold-patterned siliconoxide wafers were cut into 8 mm×8 mm slides. To minimize surfacecontaminants and unexpected scratches, the silicon oxide wafers werecoated with a layer of photoresist of 2 μm in thickness on theirpolished sides before cutting.

Surface Engineering Silicon Substrates. Silicon substrates were washedwith acetone, ethanol, and DI water before being placed in NANOSTRIP 2Xat room temperature for 30 minutes, followed by an extensive rinse withDI water and passive drying under nitrogen. The substrates were thenreacted with M-PEG-silane according to the following procedure. TheM-PEG-silane solution was prepared in nitrogen-filled reaction flasks byadding 3 mM PEG-silane in anhydrous toluene containing 1% triethylamineas catalyst. The PEG reaction proceeded under nitrogen at 60° C. for 18hours. Loosely bound moieties were removed from the PEG-treated surfacesby sonicating them in toluene and ethanol for 5 minutes each, followedby rinsing with DI water and drying under nitrogen.

Gold-Patterned Silicon Substrate. The gold regions of thepiranha-treated substrates were first reacted with a 20 mM mixture ofalkane thiols of 11-mercaptoundecanoic acid (MUA) and3-mercaptopropionic acid (MPA) (1:10 v/v) for 16 hours to form aself-assembly monolayer (SAM). The silicon background was passivatedwith PEG through the procedure described above. The substrates were thenimmersed in an aqueous solution of 150 mM EDAC and 30 mMN-hydroxysuccinimide (NHS) for 30 minutes to attach the NHS group to the—COOH terminus of SAM. The substrates with NHS on the gold and PEG onthe silicon oxide were sterilized with 70% ethanol for 15 minutes andexposed to fibronectin protein at a concentration of 0.1 mg mL⁻¹ in aphosphate buffer solution (PBS) of pH 8.2 at room temperature for 45minutes. To remove loosely bound moieties after each step of the surfacemodification, the substrate was rinsed with its original solvent and DIwater, respectively. As a result, the immobilized protein formed abiocompatible layer on the gold arrays, and the M-PEG-silane formed aninert, biocompatible layer on the silicon oxide background, as shown inFIG. 1B.

Fluorescence Labeling of Proteins. Fibronectin (M_(w)=440 kDa) at aconcentration of 1 mg/mL in PBS (pH 8.3) was reacted with Cy3monoreactive NHS ester (M_(w)=765.95 Da, 10 mg/mL in dimethylformamide)at a 100:1 ratio of dye to protein. The reaction proceeded in the darkfor 30 minutes at room temperature with gentle stirring every 10minutes. The unconjugated dye was separated by dialysis against PBSovernight at 4° C. using a Slide-A-Lyzer (Pierce Biotechnology, IL)membrane (exclusion limit of M_(r)=3500). Samples were diluted with PBSto a 0.1 mg/mL concentration, verified with UV spectroscopy beforeapplication to the surfaces. The UV absorbance of the solution diluted4-fold was 0.40225 and 1.974 AU at 280 and 548 nm, respectively.Considering a molar extinction coefficient of 150 000 M⁻¹ cm⁻¹ for Cy3dye and 563 200 M⁻¹ cm⁻¹ for fibronectin, a labeling ratio of 30.33([Cy3]/[fibronectin]) was detected.

Cell Culture. Mouse macrophage (RAW 264.7) cell line was cultured in 75cm² flasks at 37° C. in a humidified atmosphere with 5% CO₂. The mediumcontained 10% fetal bovine serum (FBS) in RPMI-1640 supplemented with 2mM L-glutamine 50 IU mL⁻¹ penicillin, and 50 μg mL⁻¹ streptomycin. Themedium was changed every third day. For cell adhesion, 0.5 mL ofmacrophage cells at a concentration of 2×10⁵ cells mL⁻¹ was plated ontothe protein-patterned substrates. The cells were allowed to adhere for3, 7, and 10 days under the standard culture condition. The adheredcells were fixed with 2% glutaraldehyde for 20 minutes at roomtemperature.

X-ray Photoelectron Spectroscopy (XPS). XPS spectra were taken on aSurface Science Instruments S-probe spectrometer. This instrument has amonochromatized A1 Kα X-ray source. The X-ray spot size for theseacquisitions was on the order of 800 μm. Pressure in the analyticalchamber during spectral acquisition was less than 5×10⁻⁹ torr. The passenergy for survey spectra (composition) was 150 eV, and that forhigh-resolution C_(1s) (HRC) and Si_(2p) (HRSi) scans was 50 eV. Thetakeoff angle (the angle between the sample normal and the input axis ofthe energy analyzer) was 55° (≅50 Å sampling depth).

The Service Physics ESCAVB Graphics Viewer program was used to determinepeak areas, calculate the elemental compositions from peak areas, andpeak-fit the high resolution spectra. The binding energy scale of thehigh-resolution C_(1s) spectra, shown in FIGS. 2F-2H, was calibrated byassigning the hydrocarbon peak in the C_(1s) high-resolution spectrum abinding energy of 285.0 eV. The binding energy scale for the Si_(2p)high-resolution spectra, shown in FIGS. 2A-2E, was calibrated to theC_(1s) peak position in the survey scan.

Contact Angle Measurements. Contact angles were measured by the sessiledrop technique using a Rame-Hart 100 goniometer under ambient laboratoryconditions (˜40% humidity). A 2 μL drop of distilled water was appliedto the surface, and the contact angle measurements were made within 30seconds of the contact. The measurements were repeated for five samples.

Differential Interference Contrast (DIC) Reflectance Microscopy.Cell-patterned surfaces were characterized with a differentialinterference contrast (DIC) reflectance microscope (Nikon E800 UprightMicroscope, New York, N.Y.). Surfaces were visualized using a DIC-10×(N. A. 0.3) and DIC-50× (N. A. 0.8) objectives. Images were acquired bya CoolSNAP camera (series A99G81021, Roper Scientific Inc., AZ) attachedto the microscope and a computer. FIGS. 4A-4I show the DIC reflectancemicroscopic images of cells cultured on the array for up to 10 days.

Fluorescence Microscopy. Fluorescence images were acquired on a NikonEclipse E800 upright wide field fluorescent microscope (NikonInstruments, Inc., Melville, N.Y.) equipped with a Photometrics CoolSNAPHQ CCD camera (Roper Scientific, Inc., Tucson, Ariz.). Surfaces werevisualized using a DIC-10× (0.46) objective and rhodamine filter(excitation, 530-560 nm; emission, 590-650 nm). The amount of proteinsadsorbed on surfaces was quantified by fluorescence intensitymeasurements. To avoid the interference from gold electrodes, thesubstrates without gold patterns were used. A rectangular region ofinterest (ROI) was selected on each image, and intensity per area wascalculated and presented in arbitrary units. FIGS. 3A-3F show thefluorescent images of fibronectin-Cy3 conjugate adsorbed on surfacespatterned with gold electrodes.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An array for guided cellpatterning, comprising a plurality of individually immobilized cellsisolated on an inert surface resistant to cell adhesion, wherein theinert surface comprises a silicon oxide surface having polyalkyleneoxide moieties covalently coupled thereto.
 2. The array of claim 1,wherein the silicon oxide surface comprises from about 40% to 65% byweight Si, from about 5% to about 20% by weight SiO_(x<2), and fromabout 20% to about 40% by weight SiO₂.
 3. The array of claim 1, whereinthe silicon oxide surface comprises from about 50% to 60% by weight Si,from about 10% to about 15% by weight SiO_(x<2), and from about 25% toabout 35% by weight SiO₂.
 4. The array of claim 1, wherein the siliconoxide surface comprises about 58% by weight Si, about 12% by weightSiO_(x<2), and about 30% by weight SiO₂.
 5. The array of claim 1,wherein each cell is immobilized through the interaction of one or moreligands and one or more cell surface receptors of the immobilized cell.6. The array of claim 5, wherein the one or more ligands are covalentlycoupled to a self-assembly monolayer on a metal surface on the inertsurface.
 7. The array of claim 5, wherein the one or more ligands areadsorbed to a self-assembly monolayer on a metal surface on the inertsurface.
 8. The array of claim 5, wherein the one or more ligands arecell adhesion peptides.
 9. The array of claim 8, wherein the celladhesion peptides are selected from the group consisting of a peptidecapable of binding human umbilical cord vein endothelial cells, apeptide capable of binding DAOY cells, and a peptide capable of bindingglioma cells.
 10. The array of claim 8, wherein the cell adhesionpeptide is Lys-Arg-Glu-Asp-Val-Tyr (SEQ ID NO.: 1).
 11. The array ofclaim 8, wherein the cell adhesion peptide is Lys-Arg-Gly-Asp (SEQ IDNO: 2).
 12. The array of claim 8, wherein the cell adhesion peptide isArg-Glu-Asp-Val-Tyr (SEQ ID NO: 3).
 13. The array of claim 8, whereinthe cell adhesion peptide is Arg-Gly-Asp.
 14. The array of claim 5,wherein the one or more ligands are peptides.
 15. The array of claim 14,wherein the peptide is chlorotoxin.
 16. The array of claim 14, whereinthe peptide is nerve growth factor.
 17. The array of claim 5, whereinthe one or more ligands are cell adhesion proteins or fragments thereof.18. The array of claim 17, wherein the cell adhesion protein isfibronectin or a fragment thereof.